feat: backend 架构基座（copier 预生成，与功能无关的横切层）

每个生成的 backend 第一天即有完整分层结构 + 能跑的横切基础设施：
- 空层包：domain / application/services / infrastructure/db（待功能填）
- routers：health(存活/就绪) + ws(WebSocket 代理骨架)
- middleware：request_id（注入+loguru 上下文）+ access_log
- exceptions：AppException 基类 + 统一 handler（标准错误响应）
- infrastructure/db/base：engine/session/Base（SQLite WAL+busy_timeout / PG 连接池）+ get_db
- utils：ids + files（work_dir/会话目录/防穿越）
- core/logging：加文件 sink 到 {WORK_DIR}/logs（运行时建，gitignore）
- main.py：装配中间件/异常/路由
- 运行时目录（logs、{uuid} 临时）由代码创建，不入库

验证：sqlite/postgresql 两配置 ruff+mypy 全过，import 成功，7 条路由就位。

template/backend/src/middleware/access_log.py

@@ -0,0 +1,26 @@
+import time
+from collections.abc import Awaitable, Callable
+
+from loguru import logger
+from starlette.middleware.base import BaseHTTPMiddleware
+from starlette.requests import Request
+from starlette.responses import Response
+
+
+class AccessLogMiddleware(BaseHTTPMiddleware):
+    """结构化记录每个 HTTP 请求：method / path / status / 耗时。"""
+
+    async def dispatch(
+        self, request: Request, call_next: Callable[[Request], Awaitable[Response]]
+    ) -> Response:
+        start = time.perf_counter()
+        response = await call_next(request)
+        duration_ms = round((time.perf_counter() - start) * 1000, 2)
+        logger.info(
+            "access | method={} path={} status={} duration_ms={}",
+            request.method,
+            request.url.path,
+            response.status_code,
+            duration_ms,
+        )
+        return response

template/backend/src/middleware/request_id.py

@@ -0,0 +1,22 @@
+from collections.abc import Awaitable, Callable
+
+from loguru import logger
+from starlette.middleware.base import BaseHTTPMiddleware
+from starlette.requests import Request
+from starlette.responses import Response
+
+from ..utils.ids import new_uuid
+
+
+class RequestIdMiddleware(BaseHTTPMiddleware):
+    """为每个请求注入 X-Request-Id，并绑定到 loguru 上下文，贯穿全链路日志。"""
+
+    async def dispatch(
+        self, request: Request, call_next: Callable[[Request], Awaitable[Response]]
+    ) -> Response:
+        request_id = request.headers.get("X-Request-Id") or new_uuid()
+        request.state.request_id = request_id
+        with logger.contextualize(request_id=request_id):
+            response = await call_next(request)
+        response.headers["X-Request-Id"] = request_id
+        return response