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

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

backend/app/services/video_worker.py

@@ -60,8 +60,8 @@ async def process_queued_jobs(conn: duckdb.DuckDBPyConnection, lock: asyncio.Loc
             now = datetime.now(timezone.utc).replace(tzinfo=None)
             async with lock:
                 conn.execute(
-                    "UPDATE generated_videos SET status = 'failed', updated_at = ? WHERE id = ?",
-                    [now, db_id],
+                    "UPDATE generated_videos SET status = 'failed', error = ?, updated_at = ? WHERE id = ?",
+                    [str(exc), now, db_id],
                 )
             continue
 
@@ -116,14 +116,15 @@ async def process_processing_jobs(conn: duckdb.DuckDBPyConnection, lock: asyncio
 
         urls = result.get("unsigned_urls") or result.get("video_urls")
         video_url = (urls or [None])[0]
+        error_msg = result.get("error")
         now = datetime.now(timezone.utc).replace(tzinfo=None)
 
         async with lock:
             conn.execute(
                 """UPDATE generated_videos
-                   SET status = ?, video_url = ?, updated_at = ?
+                   SET status = ?, video_url = ?, error = ?, updated_at = ?
                    WHERE id = ?""",
-                [job_status, video_url, now, db_id],
+                [job_status, video_url, error_msg, now, db_id],
             )
         updated += 1
         logger.info("Video job %s → %s", db_id, job_status)