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>

backend/routers/discovery.py

@@ -170,10 +170,17 @@ def refresh_discovery(
     user_id = current_user.id
 
     def _run_discovery():
+        from datetime import datetime
         from ..database import SessionLocal
+        from sqlalchemy import text as _text
         fresh_db = SessionLocal()
         try:
             run_full_discovery(fresh_db, user_id, regions)
+            fresh_db.execute(
+                _text("UPDATE user_settings SET last_discovery_run = :now WHERE user_id = :uid"),
+                {"now": datetime.utcnow(), "uid": user_id},
+            )
+            fresh_db.commit()
         finally:
             fresh_db.close()
 
@@ -251,6 +258,23 @@ def dismiss_discovery_video(
     db.commit()
 
 
+@router.get("/status")
+def discovery_status(
+    db: Session = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    from ..models import UserSettings
+    s = db.query(UserSettings).filter_by(user_id=current_user.id).first()
+    pending = db.execute(
+        text("SELECT COUNT(*) AS n FROM discovery_queue WHERE user_id = :uid AND seen = 0"),
+        {"uid": current_user.id},
+    ).mappings().first()
+    return {
+        "last_run": s.last_discovery_run.isoformat() if s and s.last_discovery_run else None,
+        "pending_count": pending["n"] if pending else 0,
+    }
+
+
 @router.get("/community", response_model=list[dict])
 def community_shelf(
     db: Session = Depends(get_db),