Add policy configuration reference documentation

Introduce a comprehensive reference guide for ORLY policy configuration. This document outlines policy options, validation rules, access control, and debugging methods, serving as the authoritative resource for policy-related behavior.

docs/POLICY_CONFIGURATION_REFERENCE.md

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+# ORLY Policy Configuration Reference
+
+This document provides a definitive reference for all policy configuration options and when each rule applies. Use this as the authoritative source for understanding policy behavior.
+
+## Quick Reference: Read vs Write Applicability
+
+| Rule Field | Write (EVENT) | Read (REQ) | Notes |
+|------------|:-------------:|:----------:|-------|
+| `size_limit` | ✅ | ❌ | Validates incoming events only |
+| `content_limit` | ✅ | ❌ | Validates incoming events only |
+| `max_age_of_event` | ✅ | ❌ | Prevents replay attacks |
+| `max_age_event_in_future` | ✅ | ❌ | Prevents future-dated events |
+| `max_expiry_duration` | ✅ | ❌ | Requires expiration tag |
+| `must_have_tags` | ✅ | ❌ | Validates required tags |
+| `protected_required` | ✅ | ❌ | Requires NIP-70 "-" tag |
+| `identifier_regex` | ✅ | ❌ | Validates "d" tag format |
+| `tag_validation` | ✅ | ❌ | Validates tag values with regex |
+| `write_allow` | ✅ | ❌ | Pubkey whitelist for writing |
+| `write_deny` | ✅ | ❌ | Pubkey blacklist for writing |
+| `read_allow` | ❌ | ✅ | Pubkey whitelist for reading |
+| `read_deny` | ❌ | ✅ | Pubkey blacklist for reading |
+| `privileged` | ❌ | ✅ | Party-involved access control |
+| `write_allow_follows` | ✅ | ✅ | Grants **both** read AND write |
+| `follows_whitelist_admins` | ✅ | ✅ | Grants **both** read AND write |
+| `script` | ✅ | ❌ | Scripts only run for writes |
+
+---
+
+## Core Principle: Validation vs Filtering
+
+The policy system has two distinct modes of operation:
+
+### Write Operations (EVENT messages)
+- **Purpose**: Validate and accept/reject incoming events
+- **All rules apply** except `read_allow`, `read_deny`, and `privileged`
+- Events are checked **before storage**
+- Rejected events are never stored
+
+### Read Operations (REQ messages)
+- **Purpose**: Filter which stored events a user can retrieve
+- **Only access control rules apply**: `read_allow`, `read_deny`, `privileged`, `write_allow_follows`, `follows_whitelist_admins`
+- Validation rules (size, age, tags) do NOT apply
+- Scripts are NOT executed for reads
+- Filtering happens **after database query**
+
+---
+
+## Configuration Structure
+
+```json
+{
+  "default_policy": "allow|deny",
+  "kind": {
+    "whitelist": [1, 3, 7],
+    "blacklist": [4, 42]
+  },
+  "owners": ["hex_pubkey_64_chars"],
+  "policy_admins": ["hex_pubkey_64_chars"],
+  "policy_follow_whitelist_enabled": true,
+  "global": { /* Rule object */ },
+  "rules": {
+    "1": { /* Rule object for kind 1 */ },
+    "30023": { /* Rule object for kind 30023 */ }
+  }
+}
+```
+
+---
+
+## Top-Level Configuration Fields
+
+### `default_policy`
+**Type**: `string`
+**Values**: `"allow"` (default) or `"deny"`
+**Applies to**: Both read and write
+
+The fallback behavior when no specific rule makes a decision.
+
+```json
+{
+  "default_policy": "deny"
+}
+```
+
+### `kind.whitelist` and `kind.blacklist`
+**Type**: `[]int`
+**Applies to**: Both read and write
+
+Controls which event kinds are processed at all.
+
+- **Whitelist** takes precedence: If present, ONLY whitelisted kinds are allowed
+- **Blacklist**: If no whitelist, these kinds are denied
+- **Neither**: Behavior depends on `default_policy` and whether rules exist
+
+```json
+{
+  "kind": {
+    "whitelist": [0, 1, 3, 7, 30023]
+  }
+}
+```
+
+### `owners`
+**Type**: `[]string` (64-character hex pubkeys)
+**Applies to**: Policy administration
+
+Relay owners with full control. Merged with `ORLY_OWNERS` environment variable.
+
+```json
+{
+  "owners": ["4a93c5ac0c6f49d2c7e7a5b8d9e0f1a2b3c4d5e6f7a8b9c0d1e2f3a4b5c6d7e8"]
+}
+```
+
+### `policy_admins`
+**Type**: `[]string` (64-character hex pubkeys)
+**Applies to**: Policy administration
+
+Pubkeys that can update policy via kind 12345 events (with restrictions).
+
+### `policy_follow_whitelist_enabled`
+**Type**: `boolean`
+**Default**: `false`
+**Applies to**: Both read and write (when `write_allow_follows` is true)
+
+When enabled, allows `write_allow_follows` rules to grant access to policy admin follows.
+
+---
+
+## Rule Object Fields
+
+Rules can be defined in `global` (applies to all events) or `rules[kind]` (applies to specific kind).
+
+### Access Control Fields
+
+#### `write_allow`
+**Type**: `[]string` (hex pubkeys)
+**Applies to**: Write only
+**Behavior**: Exclusive whitelist
+
+When present with entries, ONLY these pubkeys can write events of this kind. All others are denied.
+
+```json
+{
+  "rules": {
+    "1": {
+      "write_allow": ["pubkey1_hex", "pubkey2_hex"]
+    }
+  }
+}
+```
+
+**Special case**: Empty array `[]` explicitly allows all writers.
+
+#### `write_deny`
+**Type**: `[]string` (hex pubkeys)
+**Applies to**: Write only
+**Behavior**: Blacklist (highest priority)
+
+These pubkeys cannot write events of this kind. **Checked before allow lists.**
+
+```json
+{
+  "rules": {
+    "1": {
+      "write_deny": ["banned_pubkey_hex"]
+    }
+  }
+}
+```
+
+#### `read_allow`
+**Type**: `[]string` (hex pubkeys)
+**Applies to**: Read only
+**Behavior**: Exclusive whitelist (with OR logic for privileged)
+
+When present with entries:
+- If `privileged: false`: ONLY these pubkeys can read
+- If `privileged: true`: These pubkeys OR parties involved can read
+
+```json
+{
+  "rules": {
+    "4": {
+      "read_allow": ["trusted_pubkey_hex"],
+      "privileged": true
+    }
+  }
+}
+```
+
+#### `read_deny`
+**Type**: `[]string` (hex pubkeys)
+**Applies to**: Read only
+**Behavior**: Blacklist (highest priority)
+
+These pubkeys cannot read events of this kind. **Checked before allow lists.**
+
+#### `privileged`
+**Type**: `boolean`
+**Default**: `false`
+**Applies to**: Read only
+
+When `true`, events are only readable by "parties involved":
+- The event author (`event.pubkey`)
+- Users mentioned in `p` tags
+
+**Interaction with `read_allow`**:
+- `read_allow` present + `privileged: true` = OR logic (in list OR party involved)
+- `read_allow` empty + `privileged: true` = Only parties involved
+- `privileged: true` alone = Only parties involved
+
+```json
+{
+  "rules": {
+    "4": {
+      "description": "DMs - only sender and recipient can read",
+      "privileged": true
+    }
+  }
+}
+```
+
+#### `write_allow_follows`
+**Type**: `boolean`
+**Default**: `false`
+**Applies to**: Both read AND write
+**Requires**: `policy_follow_whitelist_enabled: true` at top level
+
+Grants **both read and write access** to pubkeys followed by policy admins.
+
+> **Important**: Despite the name, this grants BOTH read and write access.
+
+```json
+{
+  "policy_follow_whitelist_enabled": true,
+  "rules": {
+    "1": {
+      "write_allow_follows": true
+    }
+  }
+}
+```
+
+#### `follows_whitelist_admins`
+**Type**: `[]string` (hex pubkeys)
+**Applies to**: Both read AND write
+
+Alternative to `write_allow_follows` that specifies which admin pubkeys' follows are whitelisted for this specific rule.
+
+```json
+{
+  "rules": {
+    "30023": {
+      "follows_whitelist_admins": ["curator_pubkey_hex"]
+    }
+  }
+}
+```
+
+---
+
+### Validation Fields (Write-Only)
+
+These fields validate incoming events and are **completely ignored for read operations**.
+
+#### `size_limit`
+**Type**: `int64` (bytes)
+**Applies to**: Write only
+
+Maximum total serialized event size.
+
+```json
+{
+  "global": {
+    "size_limit": 100000
+  }
+}
+```
+
+#### `content_limit`
+**Type**: `int64` (bytes)
+**Applies to**: Write only
+
+Maximum size of the `content` field.
+
+```json
+{
+  "rules": {
+    "1": {
+      "content_limit": 10000
+    }
+  }
+}
+```
+
+#### `max_age_of_event`
+**Type**: `int64` (seconds)
+**Applies to**: Write only
+
+Maximum age of events. Events with `created_at` older than `now - max_age_of_event` are rejected.
+
+```json
+{
+  "global": {
+    "max_age_of_event": 86400
+  }
+}
+```
+
+#### `max_age_event_in_future`
+**Type**: `int64` (seconds)
+**Applies to**: Write only
+
+Maximum time events can be dated in the future. Events with `created_at` later than `now + max_age_event_in_future` are rejected.
+
+```json
+{
+  "global": {
+    "max_age_event_in_future": 300
+  }
+}
+```
+
+#### `max_expiry_duration`
+**Type**: `string` (ISO-8601 duration)
+**Applies to**: Write only
+
+Maximum allowed expiry time from event creation. Events **must** have an `expiration` tag when this is set.
+
+**Format**: `P[n]Y[n]M[n]W[n]DT[n]H[n]M[n]S`
+
+**Examples**:
+- `P7D` = 7 days
+- `PT1H` = 1 hour
+- `P1DT12H` = 1 day 12 hours
+- `PT30M` = 30 minutes
+
+```json
+{
+  "rules": {
+    "20": {
+      "description": "Ephemeral events must expire within 24 hours",
+      "max_expiry_duration": "P1D"
+    }
+  }
+}
+```
+
+#### `must_have_tags`
+**Type**: `[]string` (tag names)
+**Applies to**: Write only
+
+Required tags that must be present on the event.
+
+```json
+{
+  "rules": {
+    "1": {
+      "must_have_tags": ["p", "e"]
+    }
+  }
+}
+```
+
+#### `protected_required`
+**Type**: `boolean`
+**Default**: `false`
+**Applies to**: Write only
+
+Requires events to have a `-` tag (NIP-70 protected events).
+
+```json
+{
+  "rules": {
+    "4": {
+      "protected_required": true
+    }
+  }
+}
+```
+
+#### `identifier_regex`
+**Type**: `string` (regex pattern)
+**Applies to**: Write only
+
+Regex pattern that `d` tag values must match. Events **must** have a `d` tag when this is set.
+
+```json
+{
+  "rules": {
+    "30023": {
+      "identifier_regex": "^[a-z0-9-]{1,64}$"
+    }
+  }
+}
+```
+
+#### `tag_validation`
+**Type**: `map[string]string` (tag name → regex pattern)
+**Applies to**: Write only
+
+Regex patterns for validating specific tag values. Only validates tags that are **present** on the event.
+
+> **Note**: To require a tag to exist, use `must_have_tags`. `tag_validation` only validates format.
+
+```json
+{
+  "rules": {
+    "30023": {
+      "tag_validation": {
+        "t": "^[a-z0-9-]{1,32}$",
+        "d": "^[a-z0-9-]+$"
+      }
+    }
+  }
+}
+```
+
+---
+
+### Script Configuration
+
+#### `script`
+**Type**: `string` (file path)
+**Applies to**: Write only
+
+Path to a custom validation script. **Scripts are NOT executed for read operations.**
+
+```json
+{
+  "rules": {
+    "1": {
+      "script": "/etc/orly/scripts/spam-filter.py"
+    }
+  }
+}
+```
+
+---
+
+## Policy Evaluation Order
+
+### For Write Operations
+
+```
+1. Global Rule Check (all fields apply)
+   ├─ Universal constraints (size, tags, age, etc.)
+   ├─ write_deny check
+   ├─ write_allow_follows / follows_whitelist_admins check
+   └─ write_allow check
+
+2. Kind Filtering (whitelist/blacklist)
+
+3. Kind-Specific Rule Check (same as global)
+   ├─ Universal constraints
+   ├─ write_deny check
+   ├─ write_allow_follows / follows_whitelist_admins check
+   ├─ write_allow check
+   └─ Script execution (if configured)
+
+4. Default Policy (if no rules matched)
+```
+
+### For Read Operations
+
+```
+1. Global Rule Check (access control only)
+   ├─ read_deny check
+   ├─ write_allow_follows / follows_whitelist_admins check
+   ├─ read_allow check
+   └─ privileged check (party involved)
+
+2. Kind Filtering (whitelist/blacklist)
+
+3. Kind-Specific Rule Check (access control only)
+   ├─ read_deny check
+   ├─ write_allow_follows / follows_whitelist_admins check
+   ├─ read_allow + privileged (OR logic)
+   └─ privileged-only check
+
+4. Default Policy (if no rules matched)
+
+NOTE: Scripts are NOT executed for read operations
+```
+
+---
+
+## Common Configuration Patterns
+
+### Private Relay (Whitelist Only)
+
+```json
+{
+  "default_policy": "deny",
+  "global": {
+    "write_allow": ["trusted_pubkey_1", "trusted_pubkey_2"],
+    "read_allow": ["trusted_pubkey_1", "trusted_pubkey_2"]
+  }
+}
+```
+
+### Open Relay with Spam Protection
+
+```json
+{
+  "default_policy": "allow",
+  "global": {
+    "size_limit": 100000,
+    "max_age_of_event": 86400,
+    "max_age_event_in_future": 300
+  },
+  "rules": {
+    "1": {
+      "script": "/etc/orly/scripts/spam-filter.sh"
+    }
+  }
+}
+```
+
+### Community Relay (Follows-Based)
+
+```json
+{
+  "default_policy": "deny",
+  "policy_admins": ["community_admin_pubkey"],
+  "policy_follow_whitelist_enabled": true,
+  "global": {
+    "write_allow_follows": true
+  }
+}
+```
+
+### Encrypted DMs (Privileged Access)
+
+```json
+{
+  "rules": {
+    "4": {
+      "description": "Encrypted DMs - only sender/recipient",
+      "privileged": true,
+      "protected_required": true
+    }
+  }
+}
+```
+
+### Long-Form Content with Validation
+
+```json
+{
+  "rules": {
+    "30023": {
+      "description": "Long-form articles",
+      "size_limit": 100000,
+      "content_limit": 50000,
+      "max_expiry_duration": "P30D",
+      "identifier_regex": "^[a-z0-9-]{1,64}$",
+      "tag_validation": {
+        "t": "^[a-z0-9-]{1,32}$"
+      }
+    }
+  }
+}
+```
+
+---
+
+## Important Behaviors
+
+### Whitelist vs Blacklist Precedence
+
+1. **Deny lists** (`write_deny`, `read_deny`) are checked **first** and have highest priority
+2. **Allow lists** are exclusive when populated - ONLY listed pubkeys are allowed
+3. **Deny-only configuration**: If only deny list exists (no allow list), all non-denied pubkeys are allowed
+
+### Empty Arrays vs Null
+
+- `[]` (empty array explicitly set) = Allow all
+- `null` or field omitted = No list configured, use other rules
+
+### Global Rules Are Additive
+
+Global rules are always evaluated **in addition to** kind-specific rules. They cannot be overridden at the kind level.
+
+### Implicit Kind Whitelist
+
+When rules are defined but no explicit `kind.whitelist`:
+- If `default_policy: "allow"`: All kinds allowed
+- If `default_policy: "deny"` or unset: Only kinds with rules allowed
+
+---
+
+## Debugging Policy Issues
+
+Enable debug logging to see policy decisions:
+
+```bash
+export ORLY_LOG_LEVEL=debug
+```
+
+Log messages include:
+- Policy evaluation steps
+- Rule matching
+- Access decisions with reasons
+
+---
+
+## Source Code Reference
+
+- Policy struct definition: `pkg/policy/policy.go:75-144` (Rule struct)
+- Policy struct definition: `pkg/policy/policy.go:380-412` (P struct)
+- Check evaluation: `pkg/policy/policy.go:1260-1595` (checkRulePolicy)
+- Write handler: `app/handle-event.go:114-138`
+- Read handler: `app/handle-req.go:420-438`