refactor: 共享请求管道、按需懒加载、流式内存护栏与连接池治理

- 抽取 ApiRequestPipeline 单例，44 个路由文件共享同一实例
- Handler/Adapter 模块级 __getattr__ 延迟导入，减少启动时间
- 新增 ensure_stream_buffer_limit() 流式内存护栏（16MB 单行 / 32MB 总量）
- HTTP 空闲连接清理与 curl_cffi LRU 会话池
- ensure_providers_bootstrapped 按需引导指定 provider_types
- Usage 事件序列化迁移至 msgpack，Redis codec 隔离
- 启动预热任务（/readyz 就绪门控）与优雅关闭
- 通知邮件模块独立开关与 SMTP 配置校验
- CryptoService DCL 线程安全修复
- 通知模块开关 DB 查询 30s 内存缓存
- /readyz 对 unknown 状态返回 503
- 预热关闭 5s 超时保护
- 预热适配器逐个 try-except 容错
- FormatConversionRegistry 哨兵模式防并发重复物化
- 流式缓冲检查无条件执行

Closes #230

Co-authored-by: AAEE86 <ppk0227@hotmail.com>

tests/unit/test_http_client_pool_cleanup.py

@@ -0,0 +1,59 @@
+from __future__ import annotations
+
+import time
+
+import pytest
+
+from src.clients.http_client import HTTPClientPool
+
+
+class _DummyClient:
+    def __init__(self, *, is_closed: bool = False) -> None:
+        self.is_closed = is_closed
+        self.close_calls = 0
+
+    async def aclose(self) -> None:
+        self.close_calls += 1
+        self.is_closed = True
+
+
+@pytest.mark.asyncio
+async def test_cleanup_idle_clients_closes_stale_entries(monkeypatch: pytest.MonkeyPatch) -> None:
+    now = time.time()
+    stale_proxy = _DummyClient()
+    active_proxy = _DummyClient()
+    already_closed_proxy = _DummyClient(is_closed=True)
+    stale_tunnel = _DummyClient()
+
+    monkeypatch.setattr(
+        HTTPClientPool,
+        "_proxy_clients",
+        {
+            "stale": (stale_proxy, now - 1200),
+            "active": (active_proxy, now - 10),
+            "closed": (already_closed_proxy, now - 1200),
+        },
+        raising=False,
+    )
+    monkeypatch.setattr(
+        HTTPClientPool,
+        "_tunnel_clients",
+        {"tunnel-stale": (stale_tunnel, now - 1200)},
+        raising=False,
+    )
+
+    stats = await HTTPClientPool.cleanup_idle_clients(max_idle_seconds=600)
+
+    assert stats["proxy_closed"] == 1
+    assert stats["tunnel_closed"] == 1
+    assert stats["proxy_already_closed"] == 1
+    assert stats["tunnel_already_closed"] == 0
+
+    assert stale_proxy.close_calls == 1
+    assert stale_tunnel.close_calls == 1
+    assert active_proxy.close_calls == 0
+
+    assert "active" in HTTPClientPool._proxy_clients
+    assert "stale" not in HTTPClientPool._proxy_clients
+    assert "closed" not in HTTPClientPool._proxy_clients
+    assert "tunnel-stale" not in HTTPClientPool._tunnel_clients