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/plugins/proxylist_download.py

@@ -5,6 +5,7 @@ from app.core.log import logger
 
 
 class ProxyListDownloadPlugin(BaseHTTPPlugin):
+    default_config = {"max_pages": 5}
     name = "proxylist_download"
     display_name = "ProxyListDownload"
     description = "从 ProxyListDownload API 获取代理"