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>

openapi.yaml

@@ -124,7 +124,8 @@ paths:
       description: |
         Send a temporary notification with optional custom display properties.
         Designed for Home Assistant integration. Notifications auto-expire after
-        the specified duration.
+        the specified duration. Rapid calls are buffered and played sequentially —
+        each notification waits for the previous to expire before displaying.
       operationId: notify
       requestBody:
         required: false
@@ -365,7 +366,15 @@ components:
         id:
           type: string
           description: Generated event ID
-          example: "ha_notify_1704067200000"
+          example: "notify_1704067200_1"
+        queued:
+          type: boolean
+          description: True if the notification was buffered (another is currently playing)
+          example: false
+        notify_queue_size:
+          type: integer
+          description: Number of notifications now waiting in the queue
+          example: 0
         current_state:
           $ref: "#/components/schemas/Status"
 
@@ -424,6 +433,10 @@ components:
           type: array
           items:
             $ref: "#/components/schemas/ActiveEvent"
+        notify_queue_size:
+          type: integer
+          description: Number of notifications waiting in the queue
+          example: 0
         last_updated:
           type: string
           format: date-time