Feat: notification queue — buffer rapid /notify calls, play sequentially

When multiple /notify calls arrive in quick succession, they now queue up
and display one at a time rather than clobbering each other. Each notification
plays for its full duration before the next is promoted.

- /notify returns `queued: true` and `notify_queue_size` when buffered
- Cleanup thread auto-advances the queue when the playing notification expires
- /clear on the playing notification promotes the next immediately
- /clear on a queued (not-yet-playing) notification removes it from the queue
- /clear-all also drains the queue
- Status response includes `notify_queue_size` for frontend awareness

Bump to v2.3.3. Update OpenAPI spec, README, TODO.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -84,7 +84,7 @@ All detectors support: `AGGREGATOR_URL`, `CHECK_INTERVAL`, `THRESHOLD_WARNING`,
 | `/event` | POST | Register event: `{"id": "name", "priority": 1-4, "message": "optional", "ttl": seconds}` |
 | `/clear` | POST | Clear event: `{"id": "name"}` |
 | `/clear-all` | POST | Clear all active events |
-| `/notify` | POST | Notification with optional customization (see below) |
+| `/notify` | POST | Queued notification — buffered if one is playing, auto-advances when it expires |
 | `/sleep` | POST | Enter sleep mode |
 | `/wake` | POST | Exit sleep mode |
 | `/stream` | GET | SSE stream — pushes state JSON on every change (used by the frontend) |
@@ -105,6 +105,8 @@ All detectors support: `AGGREGATOR_URL`, `CHECK_INTERVAL`, `THRESHOLD_WARNING`,
 }
 ```
 
+Rapid calls are buffered — each notification plays for its full `duration` before the next is shown. `/clear-all` also drains the queue.
+
 | Field | Required | Description |
 |-------|----------|-------------|
 | `message` | No | Text to display below emote |