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/services/source_service.py

@@ -151,14 +151,19 @@ class SourceService:
         settings = await self._load_settings()
         return self._build_record(source, artist, settings)
 
-    async def list(self, artist_id: int | None = None) -> list[SourceRecord]:
-        stmt = (
-            select(Source, Artist)
-            .join(Artist, Artist.id == Source.artist_id)
-            .order_by(Artist.name.asc(), Source.id.asc())
-        )
+    async def list(
+        self, artist_id: int | None = None, failing: bool = False,
+    ) -> list[SourceRecord]:
+        stmt = select(Source, Artist).join(Artist, Artist.id == Source.artist_id)
         if artist_id is not None:
             stmt = stmt.where(Source.artist_id == artist_id)
+        if failing:
+            # Worst-first so the rollup card surfaces the loudest failures.
+            stmt = stmt.where(Source.consecutive_failures > 0).order_by(
+                Source.consecutive_failures.desc(), Artist.name.asc(),
+            )
+        else:
+            stmt = stmt.order_by(Artist.name.asc(), Source.id.asc())
         rows = (await self.session.execute(stmt)).all()
         settings = await self._load_settings()
         return [self._build_record(s, a, settings) for s, a in rows]