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>

tests/test_api_sources.py

@@ -1,7 +1,7 @@
 import pytest
 
 from backend.app import create_app
-from backend.app.models import Artist
+from backend.app.models import Artist, Source
 
 pytestmark = pytest.mark.integration
 
@@ -25,6 +25,37 @@ async def artist(db):
     return a
 
 
+@pytest.mark.asyncio
+async def test_list_failing_filter_returns_only_failing(client, artist, db):
+    healthy = Source(
+        artist_id=artist.id, platform="patreon", url="https://p/ok",
+        enabled=True, consecutive_failures=0,
+    )
+    broken = Source(
+        artist_id=artist.id, platform="pixiv", url="https://p/bad",
+        enabled=True, consecutive_failures=3, last_error="boom",
+    )
+    db.add_all([healthy, broken])
+    await db.commit()
+
+    resp = await client.get("/api/sources?failing=true")
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    assert len(body) == 1
+    assert body[0]["consecutive_failures"] == 3
+    assert body[0]["last_error"] == "boom"
+
+
+@pytest.mark.asyncio
+async def test_schedule_status_shape(client):
+    resp = await client.get("/api/sources/schedule-status")
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    assert set(body) == {"last_tick_at", "next_due_at", "due_now", "auto_sources"}
+    assert isinstance(body["due_now"], int)
+    assert isinstance(body["auto_sources"], int)
+
+
 @pytest.mark.asyncio
 async def test_create_list_get_delete(client, artist):
     create = await client.post("/api/sources", json={