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/models/trigger.py

@@ -56,7 +56,7 @@ class AutomationTrigger(BaseModel, TimestampMixin):
 
     # Precomputed next fire moment in UTC; advanced after each fire by the
     # schedule tick. NULL means the trigger has never been scheduled (the
-    # tick self-heals on first sight). Manual triggers leave this NULL.
+    # tick self-heals on first sight).
     next_fire_at = Column(TIMESTAMP(timezone=True), nullable=True)
 
     automation = relationship("Automation", back_populates="triggers")