Fix popular task failing silently when table doesn't exist

The outer try had no except — any exception (e.g. table missing) killed
the whole background task with no error visible to the user. Now:
- CREATE TABLE IF NOT EXISTS inline so the task works even if the
  startup migration hasn't run (no server restart required)
- Wrap DELETE in its own try/except
- Catch and print outer exceptions so failures appear in server logs

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

backend/routers/channels.py

@@ -719,11 +719,28 @@ def _fetch_popular_task(channel_id: int, youtube_channel_id: str):
     # Phase 1: store with basic info and write popular ranks immediately
     db = SessionLocal()
     try:
+        # Ensure table exists regardless of whether startup migration ran
+        db.execute(text("""
+            CREATE TABLE IF NOT EXISTS channel_popular_videos (
+                id INTEGER PRIMARY KEY AUTOINCREMENT,
+                channel_id INTEGER NOT NULL REFERENCES channels(id) ON DELETE CASCADE,
+                video_id INTEGER NOT NULL REFERENCES videos(id) ON DELETE CASCADE,
+                rank INTEGER NOT NULL,
+                fetched_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+                UNIQUE(channel_id, video_id)
+            )
+        """))
+        db.commit()
+
         channel = db.query(Channel).filter_by(id=channel_id).first()
         if not channel:
             return
+
+        try:
             db.execute(text("DELETE FROM channel_popular_videos WHERE channel_id = :cid"), {"cid": channel_id})
             db.commit()
+        except Exception:
+            db.rollback()
 
         for rank, entry in enumerate(entries, start=1):
             yt_id = entry["id"]
@@ -754,6 +771,9 @@ def _fetch_popular_task(channel_id: int, youtube_channel_id: str):
                 db.commit()
             except Exception:
                 db.rollback()
+    except Exception as e:
+        print(f"[popular] task error: {e}", flush=True)
+        db.rollback()
     finally:
         db.close()