fix(mma): Change self.engine to self.engines dict for concurrent track support

- self.engine was a single ConductorEngine reference that got overwritten
  when multiple tracks ran concurrently, orphaning the first track's engine
- Now uses self.engines: Dict[str, ConductorEngine] keyed by track.id
- Updated _spawn_worker, kill_worker, pause_mma, resume_mma, approve_ticket,
  _load_active_tickets, and _update_ticket_depends_on to use engines.get(track_id)

Fixes concurrent MMA track execution bug where only one worker ever appeared.

fix_engines.py

@@ -0,0 +1,12 @@
+with open('src/app_controller.py', 'rb') as f:
+    content = f.read()
+
+# Fix _update_ticket_depends_on
+old = b'    if self.engine:\r\n     from src.dag_engine import TrackDAG, ExecutionEngine\r\n     self.engine.dag = TrackDAG(self.active_track.tickets)\r\n     self.engine.engine = ExecutionEngine(self.engine.dag, auto_queue=self.engine.engine.auto_queue)'
+new = b'    engine = self.engines.get(self.active_track.id if self.active_track else None)\r\n    if engine:\r\n     from src.dag_engine import TrackDAG, ExecutionEngine\r\n     engine.dag = TrackDAG(self.active_track.tickets)\r\n     engine.engine = ExecutionEngine(engine.dag, auto_queue=engine.engine.auto_queue)'
+content = content.replace(old, new)
+
+with open('src/app_controller.py', 'wb') as f:
+    f.write(content)
+
+print("Done")