refactor(backend): optimize database safety, validator performance, and scheduler concurrency

- Fix SQL injection risks in proxy_repo and task_repo
- Atomic acquire_pending with UPDATE ... RETURNING
- Reuse aiohttp ClientSession in ValidatorService
- Replace polling with asyncio.Event in SchedulerService
- Optimize ValidationQueue.drain with asyncio.Condition
- Concurrent plugin crawling with asyncio.gather
- Unify ProxyRaw model import path
- Fix test baseline and remove tracked __pycache__ files

app/api/lifespan.py

@@ -38,4 +38,5 @@ async def lifespan(app: FastAPI):
 
     # 关闭调度器
     await scheduler_service.stop()
+    await scheduler_service.validation_queue.validator.close()
     logger.info("API server shutdown")