refactor: 30-day defaults, time-based message expiry

- Change QUEUE_MAX_AGE default from 48h to 30 days (2592000s)
- Replace count-based MAX_HISTORY with time-based MESSAGE_MAX_AGE
  (default 30 days) — messages older than the configured age are
  pruned instead of keeping a fixed count per target
- Update README to reflect new defaults and time-based expiry

README.md

@@ -249,8 +249,8 @@ Key properties:
 - **Ordered**: Queue entries have monotonically increasing IDs. Messages are
   always delivered in order within a client's queue.
 - **No delivery/read receipts** for channel messages. DM receipts are planned.
-- **Queue depth**: Server-configurable via `QUEUE_MAX_AGE`. Default is 48
-  hours. Entries older than this are pruned.
+- **Queue depth**: Server-configurable via `QUEUE_MAX_AGE`. Default is 30
+  days. Entries older than this are pruned.
 
 ### Long-Polling
 
@@ -1788,10 +1788,10 @@ skew issues) and simpler than UUIDs (integer comparison vs. string comparison).
 
 ### Data Lifecycle
 
-- **Messages**: Rotated per `MAX_HISTORY` — oldest messages beyond the limit
-  are pruned periodically per target (channel or DM).
+- **Messages**: Pruned automatically when older than `MESSAGE_MAX_AGE`
+  (default 30 days).
 - **Queue entries**: Pruned automatically when older than `QUEUE_MAX_AGE`
-  (default 48h).
+  (default 30 days).
 - **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 +1812,9 @@ 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 target (channel or DM) before rotation |
+| `MESSAGE_MAX_AGE`  | int     | `2592000`                            | Maximum age of messages in seconds (30 days). Messages older than this are pruned. |
 | `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. |
+| `QUEUE_MAX_AGE`    | int     | `2592000`                            | Maximum age of client queue entries in seconds (30 days). 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` |
@@ -2229,7 +2229,7 @@ GET /api/v1/challenge
 
 - [ ] **Hashcash proof-of-work** for session creation (abuse prevention)
 - [x] **Queue pruning** — delete old queue entries per `QUEUE_MAX_AGE`
-- [x] **Message rotation** — enforce `MAX_HISTORY` per target
+- [x] **Message expiry** — delete messages older than `MESSAGE_MAX_AGE`
 - [ ] **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)