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

tests/integration/test_proxies_api.py

@@ -121,14 +121,14 @@ class TestProxiesAPI:
         """测试 GET /api/proxies/export/csv"""
         response = await client.get("/api/proxies/export/csv")
         assert response.status_code == 200
-        assert response.headers["content-type"] == "text/csv"
+        assert response.headers["content-type"].startswith("text/csv")
 
     @pytest.mark.asyncio
     async def test_export_proxies_txt(self, client):
         """测试 GET /api/proxies/export/txt"""
         response = await client.get("/api/proxies/export/txt")
         assert response.status_code == 200
-        assert response.headers["content-type"] == "text/plain"
+        assert response.headers["content-type"].startswith("text/plain")
 
     @pytest.mark.asyncio
     async def test_export_proxies_json(self, client):