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/downloads.py

@@ -126,6 +126,54 @@ async def downloads_stats():
     return jsonify(out)
 
 
+@downloads_bp.route("/activity", methods=["GET"])
+async def downloads_activity():
+    """Hourly download-event counts over the last `?hours=` (default 24).
+
+    Returns a fixed-length, oldest-first bucket array so the UI can render
+    a sparkline directly. Bucketing is done in Python against UTC to dodge
+    session-timezone ambiguity in SQL date_trunc.
+    """
+    try:
+        hours = int(request.args.get("hours", "24"))
+    except ValueError:
+        return jsonify({"error": "invalid_hours"}), 400
+    hours = max(1, min(168, hours))
+
+    now = datetime.now(UTC)
+    end = now.replace(minute=0, second=0, microsecond=0)
+    start = end - timedelta(hours=hours - 1)
+    buckets = [
+        {"hour": (start + timedelta(hours=i)).isoformat(),
+         "ok": 0, "error": 0, "other": 0, "total": 0}
+        for i in range(hours)
+    ]
+
+    async with get_session() as session:
+        rows = (await session.execute(
+            select(DownloadEvent.started_at, DownloadEvent.status)
+            .where(DownloadEvent.started_at >= start)
+        )).all()
+
+    for started_at, status in rows:
+        if started_at is None:
+            continue
+        sa = started_at if started_at.tzinfo else started_at.replace(tzinfo=UTC)
+        idx = int((sa - start).total_seconds() // 3600)
+        if not (0 <= idx < hours):
+            continue
+        b = buckets[idx]
+        if status == "ok":
+            b["ok"] += 1
+        elif status == "error":
+            b["error"] += 1
+        else:
+            b["other"] += 1
+        b["total"] += 1
+
+    return jsonify({"hours": hours, "buckets": buckets})
+
+
 @downloads_bp.route("/<int:event_id>", methods=["GET"])
 async def get_download(event_id: int):
     async with get_session() as session: