feat(scheduler): admin dashboard endpoint + per-job activity tracking

Record each scheduler job's run activity in-process (running / last result /
error / timestamps) and expose a snapshot. New admin-only GET
/api/admin/scheduler returns the per-job activity (with APScheduler next-run
times), the DB-derived work still queued (channels/videos pending, deep ETA,
live-refresh backlog) and the shared quota picture.

backend/app/main.py

@@ -26,7 +26,19 @@ from starlette.middleware.sessions import SessionMiddleware
 
 from app import auth
 from app.config import settings
-from app.routes import admin, channels, feed, health, me, playlists, quota, sync, tags, version
+from app.routes import (
+    admin,
+    channels,
+    feed,
+    health,
+    me,
+    playlists,
+    quota,
+    scheduler as scheduler_routes,
+    sync,
+    tags,
+    version,
+)
 from app.scheduler import shutdown_scheduler, start_scheduler
 
 
@@ -68,6 +80,7 @@ app.include_router(me.router)
 app.include_router(channels.router)
 app.include_router(playlists.router)
 app.include_router(admin.router)
+app.include_router(scheduler_routes.router)
 app.include_router(quota.router)
 app.include_router(version.router)