refactor(api): TOML 配置 SSOT、统一错误契约、Auth/事务加固与可观测性 (#33)

配置 SSOT（TOML + .env）
统一错误契约
Auth 与事务边界
Redis / Celery 可靠性:业务 Redis（DB/0）与 Celery broker/backend（DB/1）显式拆分；连接池、sync client
可观测性（OpenTelemetry + LGTM）

api/tests/test_content_static_http.py

@@ -0,0 +1,37 @@
+"""Content 静态页 HTTP smoke：legal 与官网主页。"""
+
+from __future__ import annotations
+
+import pytest
+from fastapi import FastAPI
+from httpx import ASGITransport, AsyncClient
+
+from app.features.content.router import router as content_router
+from tests.conftest import install_test_error_handlers
+
+
+@pytest.fixture
+def content_app() -> FastAPI:
+    app = install_test_error_handlers(FastAPI())
+    app.include_router(content_router)
+    return app
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "path,expected_fragment",
+    [
+        ("/api/legal/terms", "用户协议"),
+        ("/api/legal/privacy", "隐私"),
+        ("/", "岁月留书"),
+    ],
+)
+async def test_content_static_pages_return_html(
+    content_app: FastAPI, path: str, expected_fragment: str
+) -> None:
+    transport = ASGITransport(app=content_app)
+    async with AsyncClient(transport=transport, base_url="http://test") as client:
+        r = await client.get(path)
+    assert r.status_code == 200
+    assert r.headers.get("content-type", "").startswith("text/html")
+    assert expected_fragment in r.text