Build OverSnitch dashboard

Full implementation on top of the Next.js scaffold:

- Leaderboard with per-user request count, storage, avg GB/req, and
  optional Tautulli watch stats (plays, watch hours), each with dense
  per-metric rank (#N/total)
- SWR cache on /api/stats (5-min stale, force-refresh via button);
  client-side localStorage seed so the UI is instant on return visits
- Alerting system: content-centric alerts (unfulfilled downloads,
  partial TV downloads, stale pending requests) and user-behavior
  alerts (ghost requester, low watch rate, declined streak)
- Partial TV detection: flags ended series with <90% of episodes on disk
- Alert persistence in data/alerts.json with open/closed state,
  auto-resolve when condition clears, manual close with per-category
  cooldown, and per-alert notes
- Alert detail page rendered as a server component for instant load
- Dark UI with Tailwind v4, severity-colored left borders, summary
  cards with icons, sortable leaderboard table

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -39,3 +39,6 @@ yarn-error.log*
 # typescript
 *.tsbuildinfo
 next-env.d.ts
+
+# alert persistence
+/data/alerts.json