feat(scheduler): admin-editable job intervals (DB-backed, live reschedule)

Add scheduler_settings (per-job interval override, migration 0015) and a
PATCH /api/admin/scheduler/jobs/{id} that persists the override and live-
reschedules the running APScheduler job. Intervals load from the DB (env
defaults as fallback); the snapshot reports the live trigger interval.

backend/app/models.py

@@ -325,6 +325,17 @@ class AppState(Base):
     )
 
 
+class SchedulerSetting(Base):
+    """Admin override for a scheduler job's run interval (minutes). One row per job id;
+    absent = use the env/config default. DB-backed so it's editable from the admin UI at
+    runtime without a redeploy (env stays a fallback)."""
+
+    __tablename__ = "scheduler_settings"
+
+    job_id: Mapped[str] = mapped_column(String(40), primary_key=True)
+    interval_minutes: Mapped[int] = mapped_column(Integer)
+
+
 class Playlist(Base):
     """A per-user named, ordered collection of videos from the shared catalog.