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/persistence/enums/trigger_type.py

@@ -1,4 +1,9 @@
-"""Trigger-kind discriminator. v1: schedule | manual; webhook/event in Phase 2/3."""
+"""Trigger-kind discriminator.
+
+v1 only registers ``schedule``. ``manual`` is reserved in the enum (mirrors the
+postgres enum) but is intentionally unregistered pending a redesign of the
+"Run now" UX.
+"""
 
 from __future__ import annotations