fix: 添加 Redis 分布式锁防止 poll_video_task 重复派发

recover_stuck_tasks 在 API 超时 >3 分钟时可能重复派发同一任务，
导致重复扣费风险。通过 cache.add 实现互斥锁保护。

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/apps/generation/tasks.py

@@ -21,19 +21,29 @@ def poll_video_task(self, record_id):
     from apps.generation.models import GenerationRecord
     from utils.airdrama_client import query_task, map_status
 
+    # 防重复：同一 record 同一时刻只允许一个 poll 在执行
+    from django.core.cache import cache
+    lock_key = f'poll_lock:{record_id}'
+    if not cache.add(lock_key, '1', timeout=POLL_INTERVAL * 3):
+        logger.info('poll_video_task: record %s already being polled, skipping', record_id)
+        return
+
     try:
         record = GenerationRecord.objects.get(pk=record_id)
     except GenerationRecord.DoesNotExist:
         logger.warning('poll_video_task: record %s not found', record_id)
+        cache.delete(lock_key)
         return
 
     ark_task_id = record.ark_task_id
     if not ark_task_id:
         logger.warning('poll_video_task: record %s has no ark_task_id', record_id)
+        cache.delete(lock_key)
         return
 
     if record.status not in ('queued', 'processing'):
         logger.info('poll_video_task: record %s already in terminal state: %s', record_id, record.status)
+        cache.delete(lock_key)
         return
 
     # Poll Volcano API
@@ -42,12 +52,14 @@ def poll_video_task(self, record_id):
         new_status = map_status(ark_resp.get('status', ''))
     except Exception:
         logger.exception('poll_video_task: API query failed for %s, will retry', ark_task_id)
+        cache.delete(lock_key)
         raise self.retry(countdown=POLL_INTERVAL)
 
     if new_status in ('queued', 'processing'):
         # Still running — update status, then re-enqueue
         record.status = new_status
         record.save(update_fields=['status', 'updated_at'])
+        cache.delete(lock_key)
         raise self.retry(countdown=POLL_INTERVAL)
 
     # Terminal state reached — process result