refactor(automations): park manual trigger pending Run-now redesign

Manual-as-a-standalone-trigger conflates "user clicks Run now" with the
trigger model and forces ad-hoc input plumbing on the caller. Remove the
unreachable surface so the tree reflects reality (schedule is the only
v1 trigger).

- Unregister `manual`: drop import from triggers/__init__.py
- Delete `app/automations/triggers/manual/`
- Drop `RunService.dispatch_manual` (RunService is now read-only)
- Drop `POST /automations/{id}/run` and `RunDispatched` schema
- Keep `TriggerType.MANUAL` Python + PG enum value (reserved, documented)
  to avoid an Alembic round-trip when Run-now is redesigned

surfsense_backend/app/automations/triggers/__init__.py

@@ -1,6 +1,6 @@
 """Triggers domain: registry surface + built-in trigger packages.
 
-Each trigger lives in its own subpackage (``manual/``, ``schedule/``, ...) and
+Each trigger lives in its own subpackage (``schedule/``, ...) and
 self-registers at import time via its ``definition`` module.
 """
 
@@ -17,4 +17,4 @@ __all__ = [
 ]
 
 # Built-in triggers self-register at import time.
-from . import manual, schedule  # noqa: E402, F401
+from . import schedule  # noqa: E402, F401