Add hooks system for external triggers during G-code execution

- New Hooks module (src/py/bbctrl/Hooks.py) that watches controller state
  and fires webhooks or scripts on events:
  - tool-change (M6), program-start, program-end, pause, estop,
    homing-start, homing-end, custom (via MSG comments)
- API endpoints:
  - GET /api/hooks - get current hook config
  - PUT /api/hooks/save - save hook config
  - PUT /api/hooks/fire/<event> - manually fire a hook (for testing)
- Hook config stored in hooks.json with two types:
  - webhook: HTTP POST/PUT to external URL with JSON context
  - script: run local command with env vars (HOOK_OLD_TOOL, etc.)
- Fix tornado.web.asynchronous deprecation in Camera.py
- Wired into Ctrl initialization and state listener system

src/py/bbctrl/Camera.py

@@ -468,8 +468,7 @@ class VideoHandler(web.RequestHandler):
         self.camera = app.camera
 
 
-    @web.asynchronous
-    def get(self):
+    async def get(self):
         self.request.connection.stream.max_write_buffer_size = 10000
 
         self.set_header('Cache-Control', 'no-store, no-cache, must-revalidate, '