Popular tab now shows only flagged popular videos in rank order

Add channel_popular_videos table (channel_id, video_id, rank).
_fetch_popular_task clears and rewrites this table after each fetch.
GET /channels/{id}/videos?sort=popular now JOINs this table and orders
by rank instead of view_count, so the tab shows exactly the videos
YouTube returned in popularity order — nothing more.

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

backend/main.py

@@ -87,6 +87,14 @@ def on_startup():
             crawled_at DATETIME DEFAULT CURRENT_TIMESTAMP
         )""",
         "ALTER TABLE playlists ADD COLUMN video_ids 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)
+        )""",
         """CREATE TABLE IF NOT EXISTS search_history (
             id INTEGER PRIMARY KEY AUTOINCREMENT,
             user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,