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>

src/lib/tautulli.ts

@@ -1,4 +1,5 @@
 import { TautulliUser } from "@/lib/types";
+import { getSettings } from "@/lib/settings";
 
 interface TautulliRow {
   friendly_name: string;
@@ -22,8 +23,9 @@ interface TautulliResponse {
  * Returns null if TAUTULLI_URL/TAUTULLI_API are not set.
  */
 export async function buildTautulliMap(): Promise<Map<string, TautulliUser> | null> {
-  const url = process.env.TAUTULLI_URL;
-  const key = process.env.TAUTULLI_API;
+  const { tautulli } = getSettings();
+  const url = tautulli.url;
+  const key = tautulli.apiKey;
 
   if (!url || !key) return null;