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>

index.html

@@ -216,7 +216,7 @@
 		<script>
 			const emoteEl = document.getElementById("emote");
 			const messageEl = document.getElementById("message");
-			const VERSION = "v2.3.2";
+			const VERSION = "v2.3.3";
 
 			// Sound system
 			let audioCtx = null;