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/Web.py

@@ -766,6 +766,22 @@ class RotaryHandler(bbctrl.APIHandler):
             log.error('Unexpected error: {}'.format(e))
 
 
+class HooksGetHandler(bbctrl.APIHandler):
+    def get(self):
+        self.write_json(self.get_ctrl().hooks.get_config())
+
+
+class HooksSaveHandler(bbctrl.APIHandler):
+    def put_ok(self):
+        self.get_ctrl().hooks.save_config(self.json)
+
+
+class HooksFireHandler(bbctrl.APIHandler):
+    def put_ok(self, event):
+        data = self.json if hasattr(self, 'json') and self.json else {}
+        self.get_ctrl().hooks._fire(event, data)
+
+
 class RemoteDiagnosticsHandler(bbctrl.APIHandler):
 
     def get(self):
@@ -941,6 +957,9 @@ class Web(tornado.web.Application):
             (r'/api/time', TimeHandler),
             (r'/api/rotary', RotaryHandler),
             (r'/api/remote-diagnostics', RemoteDiagnosticsHandler),
+            (r'/api/hooks', HooksGetHandler),
+            (r'/api/hooks/save', HooksSaveHandler),
+            (r'/api/hooks/fire/([\w-]+)', HooksFireHandler),
             (r'/(.*)', StaticFileHandler,
              {'path': bbctrl.get_resource('http/'),
               'default_filename': 'index.html'}),