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>

README.md

@@ -16,6 +16,7 @@ Turn an old phone (with its OLED screen) into a glanceable ambient display for y
 - **Extensible** — Add custom detectors for any metric
 - **Personality** — Rotating expressions, celebration animations, sleep mode
 - **Sound effects** — Optional audio cues for state changes (tap to enable)
+- **Notification queue** — Rapid `/notify` bursts are buffered and played sequentially, never clobbering
 - **Home Assistant ready** — Webhook endpoints for notifications and automation
 
 ## Quick Start