Add settings UI, Discord notifications, and alert detail improvements

- Settings modal (gear icon) lets you configure all service URLs and API
  keys from the dashboard; values persist to data/settings.json with
  process.env as fallback so existing .env.local setups keep working
- Per-service Test button hits each service's status endpoint and reports
  the version on success
- Discord webhook support: structured embeds per alert category (requesters,
  approval age, episode progress, watch-rate stats) sent on new/reopened
  alerts only — already-open alerts don't re-notify
- Alert detail page restructured: prose descriptions replaced with labelled
  fields, episode progress bar for partial TV, watch-rate stat block,
  View in Radarr/Sonarr/Seerr action buttons, requester names link to
  Overseerr profiles, timestamps moved inline with status
- Tab state is pure client state (no ?tab= in URL); router.back() used
  on alert detail for clean browser history

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

.gitignore

@@ -42,3 +42,10 @@ next-env.d.ts
 
 # alert persistence
 /data/alerts.json
+/data/alerts.json.migrated
+/data/alerts.db
+/data/alerts.db-shm
+/data/alerts.db-wal
+
+# settings (API keys — never commit)
+/data/settings.json