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>
