feat(dashboards): scheduler health strip, failing-source rollup, 24h activity sparkline, credential staleness nudge

D1 scheduler visibility: AppSetting last-tick stamp on every Beat tick +
GET /api/sources/schedule-status (last_tick_at/next_due_at/due_now/auto_sources)
+ SchedulerStatusBar on the Subscriptions tab (re-polled every 30s).

D2 failing-source rollup: ?failing=true on the sources list + FailingSourcesCard
on Downloads with per-source and bulk "retry" (re-runs the feed via /check).

D3 activity sparkline: GET /api/downloads/activity hourly buckets + CSS bar
chart by the stat chips (failures stacked in error color); refreshes on live poll.

D4 credential staleness: surface last_verified age + "re-verify recommended"
warning past 30d; also fixes the dead last_verified_at field-name mismatch so
the verification row renders at all.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/api/sources.py

@@ -5,6 +5,7 @@ from sqlalchemy import select
 
 from ..extensions import get_session
 from ..models import DownloadEvent, Source
+from ..services.scheduler_service import scheduler_status
 from ..services.source_service import (
     KNOWN_PLATFORMS,
     ArtistNotFoundError,
@@ -35,11 +36,19 @@ async def list_sources():
             artist_id = int(artist_id_raw)
         except ValueError:
             return _bad("invalid_artist_id", detail="artist_id must be an integer")
+    failing = request.args.get("failing", "").lower() in ("1", "true", "yes")
     async with get_session() as session:
-        records = await SourceService(session).list(artist_id=artist_id)
+        records = await SourceService(session).list(artist_id=artist_id, failing=failing)
     return jsonify([r.to_dict() for r in records])
 
 
+@sources_bp.route("/schedule-status", methods=["GET"])
+async def schedule_status():
+    """FC-dashboards: scheduler health for the Subscriptions hub."""
+    async with get_session() as session:
+        return jsonify(await scheduler_status(session))
+
+
 @sources_bp.route("/<int:source_id>", methods=["GET"])
 async def get_source(source_id: int):
     async with get_session() as session: