Fix plugin list bug and switch watcher to PollingObserver

- Fix: list_plugins imported _REGISTRY as a direct reference to the
  empty list that existed at import time; register_services() replaces
  _REGISTRY with a new list so the imported reference was always [].
  Added get_registry() helper so callers access the live list via the
  module namespace. GET /api/plugins now correctly returns accessible
  plugins for the current user.

- Fix: switch watchdog from InotifyObserver to PollingObserver. Inotify
  events from the macOS host are not forwarded through the Docker bind
  mount, so new files were only detected via the startup scan. PollingObserver
  (1s default interval) works reliably on all platforms including
  macOS+Docker bind mounts.

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

backend/app/services/service_health.py

@@ -162,3 +162,8 @@ def get_service_url(service_id: str) -> str | None:
         if svc.id == service_id:
             return svc.internal_url
     return None
+
+
+def get_registry() -> list[ServiceDefinition]:
+    """Return the current service registry (always up-to-date after register_services)."""
+    return _REGISTRY