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
@@ -191,6 +192,8 @@ automation:
       service: rest_command.kao_wake
 ```
 
+Rapid `/notify` calls are queued automatically — each plays for its full `duration` before the next is shown. `/clear-all` drains the queue.
+
 **Notify options:**
 - `emote`: Any ASCII emote (e.g., `( °o°)`, `\\(^o^)/`)
 - `color`: Hex color (e.g., `#FF9900`)
@@ -225,7 +228,7 @@ Navigate with `↑↓` or `Tab`, press `Enter` to fire, `Q` to quit. A toast con
 | `/event`     | POST   | Register an event                                     |
 | `/clear`     | POST   | Clear an event by ID                                  |
 | `/clear-all` | POST   | Clear all active events                               |
-| `/notify`    | POST   | Simple notification `{"message": "", "duration": 5}` |
+| `/notify`    | POST   | Queued notification `{"message": "", "duration": 5}` — buffered if one is playing |
 | `/sleep`     | POST   | Enter sleep mode                                      |
 | `/wake`      | POST   | Exit sleep mode                                       |
 | `/docs`      | GET    | Interactive API documentation (Swagger UI)            |