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/schemas/api/run.py

@@ -1,4 +1,4 @@
-"""Response schemas for run sub-resources and run dispatch."""
+"""Response schemas for run sub-resources."""
 
 from __future__ import annotations
 
@@ -40,10 +40,3 @@ class RunList(BaseModel):
 
     items: list[RunSummary]
     total: int
-
-
-class RunDispatched(BaseModel):
-    """Response of a successful run dispatch."""
-
-    run_id: int
-    status: RunStatus