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_crawlers.py

@@ -0,0 +1,47 @@
+import asyncio
+import app.plugins
+from app.core.plugin_system.registry import registry
+from app.core.log import logger
+import logging
+logger.setLevel(logging.WARNING)
+
+async def test_plugin(p, timeout=20):
+    try:
+        proxies = await asyncio.wait_for(p.crawl(), timeout=timeout)
+        return len(proxies), proxies[:1] if proxies else []
+    except asyncio.TimeoutError:
+        return -2, []
+    except Exception as e:
+        return -1, [str(e)]
+
+async def test_all():
+    plugins = registry.list_plugins()
+    print(f'Total plugins: {len(plugins)}')
+    results = {}
+    for p in plugins:
+        print(f'Testing {p.name} (timeout=20s)...', flush=True)
+        count, sample = await test_plugin(p, timeout=20)
+        results[p.name] = count
+        if count > 0:
+            print(f'  -> OK: {count} proxies, sample={sample[0]}')
+        elif count == 0:
+            print(f'  -> EMPTY')
+        elif count == -2:
+            print(f'  -> TIMEOUT')
+        else:
+            print(f'  -> ERROR: {sample[0]}')
+    
+    print('\n' + '='*50)
+    print('SUMMARY:')
+    for name, count in sorted(results.items()):
+        if count > 0:
+            status = 'OK'
+        elif count == 0:
+            status = 'EMPTY'
+        elif count == -2:
+            status = 'TIMEOUT'
+        else:
+            status = 'ERROR'
+        print(f'  {name:22s} {status:8s} ({count} proxies)')
+
+asyncio.run(test_all())