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/Ctrl.py

@@ -59,6 +59,7 @@ class Ctrl(object):
             self.preplanner = bbctrl.Preplanner(self)
             if not args.demo: self.jog = bbctrl.Jog(self)
             self.pwr = bbctrl.Pwr(self)
+            self.hooks = bbctrl.Hooks(self)
 
             self.mach.connect()