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
