Fix discovery refresh: open fresh DB session in background task

The refresh endpoint was passing the request's db session to the
background task, which is closed before the task runs — silently
doing nothing on every refresh.

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

backend/routers/discovery.py

@@ -163,7 +163,17 @@ def refresh_discovery(
     s = db.query(UserSettings).filter_by(user_id=current_user.id).first()
     regions_str = (s.discovery_regions if s and s.discovery_regions else "US,SE")
     regions = [r.strip().upper() for r in regions_str.split(",") if r.strip()]
-    background_tasks.add_task(run_full_discovery, db, current_user.id, regions)
+    user_id = current_user.id
+
+    def _run_discovery():
+        from ..database import SessionLocal
+        fresh_db = SessionLocal()
+        try:
+            run_full_discovery(fresh_db, user_id, regions)
+        finally:
+            fresh_db.close()
+
+    background_tasks.add_task(_run_discovery)
     from .channels import _enrich_missing_task
     background_tasks.add_task(_enrich_missing_task, 20)
     return {"detail": "Discovery refresh started"}