fix: 修复爬虫网络层、验证队列卡死及 API 500 错误

- 修复 BaseHTTPPlugin 连接池、并发控制、异常日志、超时策略
- 修复/增强 8 个爬虫插件的稳定性和 fallback 机制
- 清理 validation_tasks 表 4 万+ pending 任务，避免队列卡死
- 修复 app/api/main.py 缺失全局 app 实例导致的 500 错误
- 提升前端 Axios 超时到 120 秒，避免请求断开
- 修复插件统计持久化和调度器生命周期问题

_test_batch.py

@@ -0,0 +1,18 @@
+import asyncio
+import time
+import app.plugins
+from app.services.plugin_service import PluginService
+
+async def main():
+    svc = PluginService()
+    start = time.time()
+    results = await svc.run_all_plugins()
+    elapsed = time.time() - start
+    print(f"Batch crawl completed in {elapsed:.2f}s")
+    print(f"Total unique proxies: {len(results)}")
+    from collections import Counter
+    c = Counter(p.protocol for p in results)
+    for proto, cnt in sorted(c.items()):
+        print(f"  {proto}: {cnt}")
+
+asyncio.run(main())