feat: implement queue pruning and message rotation

Enforce QUEUE_MAX_AGE and MAX_HISTORY config values that previously
existed but were not applied. The existing cleanup loop now also:

- Prunes client_queues entries older than QUEUE_MAX_AGE (default 48h)
- Rotates messages per target (channel/DM) beyond MAX_HISTORY (default 10000)
- Removes orphaned messages no longer referenced by any client queue

closes #40

README.md

@@ -1788,10 +1788,11 @@ skew issues) and simpler than UUIDs (integer comparison vs. string comparison).
 
 ### Data Lifecycle
 
-- **Messages**: Stored indefinitely in the current implementation. Rotation
-  per `MAX_HISTORY` is planned.
-- **Queue entries**: Stored until pruned. Pruning by `QUEUE_MAX_AGE` is
-  planned.
+- **Messages**: Rotated per `MAX_HISTORY` — oldest messages beyond the limit
+  are pruned periodically per target (channel or DM). Orphaned messages (no
+  longer referenced by any client queue) are also removed.
+- **Queue entries**: Pruned automatically when older than `QUEUE_MAX_AGE`
+  (default 48h).
 - **Channels**: Deleted when the last member leaves (ephemeral).
 - **Users/sessions**: Deleted on `QUIT` or `POST /api/v1/logout`. Idle
   sessions are automatically expired after `SESSION_IDLE_TIMEOUT` (default
@@ -1812,9 +1813,9 @@ directory is also loaded automatically via
 | `PORT`             | int     | `8080`                               | HTTP listen port |
 | `DBURL`            | string  | `file:///var/lib/neoirc/state.db?_journal_mode=WAL` | SQLite connection string. For file-based: `file:///path/to/db.db?_journal_mode=WAL`. For in-memory (testing): `file::memory:?cache=shared`. |
 | `DEBUG`            | bool    | `false`                              | Enable debug logging (verbose request/response logging) |
-| `MAX_HISTORY`      | int     | `10000`                              | Maximum messages retained per channel before rotation (planned) |
+| `MAX_HISTORY`      | int     | `10000`                              | Maximum messages retained per target (channel or DM) before rotation |
 | `SESSION_IDLE_TIMEOUT` | string | `24h`                            | Session idle timeout as a Go duration string (e.g. `24h`, `30m`). Sessions with no activity for this long are expired and the nick is released. |
-| `QUEUE_MAX_AGE`    | int     | `172800`                             | Maximum age of client queue entries in seconds (48h). Entries older than this are pruned (planned). |
+| `QUEUE_MAX_AGE`    | int     | `172800`                             | Maximum age of client queue entries in seconds (48h). Entries older than this are pruned. |
 | `MAX_MESSAGE_SIZE` | int     | `4096`                               | Maximum message body size in bytes (planned enforcement) |
 | `LONG_POLL_TIMEOUT`| int     | `15`                                 | Default long-poll timeout in seconds (client can override via query param, server caps at 30) |
 | `MOTD`             | string  | `""`                                 | Message of the day, shown to clients via `GET /api/v1/server` |
@@ -2228,8 +2229,8 @@ GET /api/v1/challenge
 ### Post-MVP (Planned)
 
 - [ ] **Hashcash proof-of-work** for session creation (abuse prevention)
-- [ ] **Queue pruning** — delete old queue entries per `QUEUE_MAX_AGE`
-- [ ] **Message rotation** — enforce `MAX_HISTORY` per channel
+- [x] **Queue pruning** — delete old queue entries per `QUEUE_MAX_AGE`
+- [x] **Message rotation** — enforce `MAX_HISTORY` per target
 - [ ] **Channel modes** — enforce `+i`, `+m`, `+s`, `+t`, `+n`
 - [ ] **User channel modes** — `+o` (operator), `+v` (voice)
 - [x] **MODE command** — query channel and user modes (set not yet implemented)