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/routes/scheduler.py

@@ -1,7 +1,7 @@
 """Admin Scheduler dashboard: a live view of what the background scheduler is doing —
 per-job run activity (from the in-process scheduler), the work still queued (DB-derived),
 and the shared quota picture."""
-from fastapi import APIRouter, Depends
+from fastapi import APIRouter, Depends, HTTPException
 from sqlalchemy import and_, func, or_, select
 from sqlalchemy.orm import Session
 
@@ -10,12 +10,36 @@ from app.config import settings
 from app.db import get_db
 from app.models import Channel, Subscription, User, Video
 from app.routes.admin import admin_user
-from app.scheduler import scheduler_snapshot
+from app.scheduler import JOB_INTERVALS, MAX_INTERVAL, MIN_INTERVAL, apply_interval, scheduler_snapshot
 from app.sync.runner import estimate_deep_backfill
 
 router = APIRouter(prefix="/api/admin/scheduler", tags=["admin"])
 
 
+@router.patch("/jobs/{job_id}")
+def set_job_interval(
+    job_id: str,
+    payload: dict,
+    _: User = Depends(admin_user),
+    db: Session = Depends(get_db),
+) -> dict:
+    """Change how often a scheduler job runs (minutes). Persisted as an override and applied
+    to the live scheduler immediately."""
+    if job_id not in JOB_INTERVALS:
+        raise HTTPException(status_code=404, detail="Unknown job")
+    try:
+        minutes = int(payload.get("interval_minutes"))
+    except (TypeError, ValueError):
+        raise HTTPException(status_code=400, detail="interval_minutes must be a number")
+    if not (MIN_INTERVAL <= minutes <= MAX_INTERVAL):
+        raise HTTPException(
+            status_code=400,
+            detail=f"interval must be between {MIN_INTERVAL} and {MAX_INTERVAL} minutes",
+        )
+    applied = apply_interval(db, job_id, minutes)
+    return {"id": job_id, "interval_minutes": applied}
+
+
 @router.get("")
 def get_scheduler(
     _: User = Depends(admin_user), db: Session = Depends(get_db)