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/services/trigger.py

@@ -69,7 +69,7 @@ class TriggerService:
             trigger.enabled = data["enabled"]
 
         # Recompute next_fire_at when schedule timing changed or the trigger was
-        # toggled back on. Manual triggers always have NULL next_fire_at.
+        # toggled back on.
         if trigger.type == TriggerType.SCHEDULE:
             trigger.next_fire_at = _initial_next_fire(
                 trigger.type, trigger.params, trigger.enabled