feat: task queue system with startup drain and backlogging (#76)

* feat: add task queue system for Timmy - all work goes through the queue

- Add queue position tracking to task_queue models with task_type field
- Add TaskProcessor class that consumes tasks from queue one at a time
- Modify chat route to queue all messages for async processing
- Chat responses get 'high' priority to jump ahead of thought tasks
- Add queue status API endpoints for position polling
- Update UI to show queue position (x/y) and current task banner
- Replace thinking loop with task-based approach - thoughts are queued tasks
- Push responses to user via WebSocket instead of immediate HTTP response
- Add database migrations for existing tables

* feat: Timmy drains task queue on startup, backlogs unhandleable tasks

On spin-up, Timmy now iterates through all pending/approved tasks
immediately instead of waiting for the polling loop. Tasks without a
registered handler or with permanent errors are moved to a new
BACKLOGGED status with a reason, keeping the queue clear for work
Timmy can actually do.

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

---------

Co-authored-by: Alexander Payne <apayne@MM.local>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/dashboard/templates/partials/chat_message.html

@@ -7,6 +7,11 @@
   <div class="msg-meta">TIMMY // {{ timestamp }}</div>
   <div class="msg-body timmy-md">{{ response | e }}</div>
 </div>
+{% if queue_info %}
+<div class="queue-status" data-task-id="{{ task_id }}" data-position="{{ queue_info.position }}" data-total="{{ queue_info.total }}">
+  <small class="text-muted">Position in queue: {{ queue_info.position }}/{{ queue_info.total }}</small>
+</div>
+{% endif %}
 <script>
   (function() {
     var el = document.currentScript.previousElementSibling.querySelector('.timmy-md');