feat(automations): add event trigger source, selector and registration

surfsense_backend/app/automations/triggers/builtin/event/source.py

@@ -0,0 +1,19 @@
+"""Event trigger source: the bus subscriber that enqueues the selector.
+
+Runs in whatever process published the event, so it stays thin — it only hands
+the event to a worker (the selector does the DB matching).
+"""
+
+from __future__ import annotations
+
+from app.event_bus import Event
+
+TASK_NAME = "automation_event_select"
+
+
+async def on_event(event: Event) -> None:
+    """Enqueue the selector for ``event``."""
+    # Lazy import: keeps app.celery_app out of the triggers-package import graph.
+    from app.celery_app import celery_app
+
+    celery_app.send_task(TASK_NAME, kwargs={"event": event.model_dump(mode="json")})