Auto-schedule daily discovery + fix Find More UX + expand query diversity

Auto-discovery daemon:
- Runs every hour, triggers full discovery for any user whose last run
  was >23 hours ago. First check is 5 minutes after startup.
- Tracks run time in user_settings.last_discovery_run (new column).
- Manual Find More also stamps last_discovery_run.

Discovery status endpoint (GET /api/discovery/status):
- Returns pending_count (unseen queue size) and last_run timestamp.
- Shown in the Discover page header so users know queue state at a glance.

Find More UX fix:
- Was: kick background task, wait 8 seconds, refetch (task takes minutes).
- Now: button shows "Queued ✓" on success with an explanatory banner
  telling the user it takes a few minutes and also runs daily automatically.

Query diversity:
- Added "best [category] channels" serendipity queries to crawl_by_search.
- Limit raised from 25 to 30 queries per run.

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

frontend/src/api/index.js

@@ -140,6 +140,7 @@ export const followDiscovery = (channelId) =>
 export const dismissDiscovery = (channelId) =>
   api.post(`/discovery/${channelId}/dismiss`);
 export const refreshDiscovery = () => api.post("/discovery/refresh");
+export const getDiscoveryStatus = () => api.get("/discovery/status");
 export const getCommunityShelf = () => api.get("/discovery/community");
 
 // Stats