feat(automations): add schedule trigger source, selector and inputs

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

@@ -0,0 +1,20 @@
+"""Schedule trigger source: Celery Beat ticks the selector every minute.
+
+``BEAT_SCHEDULE`` is merged into ``celery_app.conf.beat_schedule``. Per-row cron
+math is precomputed (the ``next_fire_at`` column), so each tick is an indexed
+lookup rather than N cron evaluations.
+"""
+
+from __future__ import annotations
+
+from celery.schedules import crontab
+
+TASK_NAME = "automation_schedule_select"
+
+BEAT_SCHEDULE = {
+    "automation-schedule-select": {
+        "task": TASK_NAME,
+        "schedule": crontab(minute="*"),
+        "options": {"expires": 50},
+    },
+}