feat: add video job cancellation functionality and error tracking in generated videos

Co-authored-by: Copilot <copilot@github.com>

backend/app/db.py

@@ -121,6 +121,9 @@ def _run_migrations(conn: duckdb.DuckDBPyConnection) -> None:
     conn.execute("""
         ALTER TABLE generated_videos ADD COLUMN IF NOT EXISTS generation_type VARCHAR DEFAULT 'text_to_video'
     """)
+    conn.execute("""
+        ALTER TABLE generated_videos ADD COLUMN IF NOT EXISTS error VARCHAR
+    """)
     _seed_admin(conn)