feat: implement WebDAV protocol support with WsgiDAV + account management API

Add complete WebDAV support: management REST API (CRUD accounts at /api/v1/webdav/accounts)
and DAV protocol endpoint (/dav) using WsgiDAV + a2wsgi bridge for client access via
HTTP Basic Auth. Includes Redis+TTLCache auth caching and integration tests (24 cases).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

routers/dav/__init__.py

@@ -0,0 +1,35 @@
+"""
+WebDAV 协议入口
+
+使用 WsgiDAV + a2wsgi 提供 WebDAV 协议支持。
+WsgiDAV 在 a2wsgi 的线程池中运行，不阻塞 FastAPI 事件循环。
+"""
+from a2wsgi import WSGIMiddleware
+from wsgidav.wsgidav_app import WsgiDAVApp
+
+from .domain_controller import DiskNextDomainController
+from .provider import DiskNextDAVProvider
+
+_wsgidav_config: dict[str, object] = {
+    "provider_mapping": {
+        "/": DiskNextDAVProvider(),
+    },
+    "http_authenticator": {
+        "domain_controller": DiskNextDomainController,
+        "accept_basic": True,
+        "accept_digest": False,
+        "default_to_digest": False,
+    },
+    "verbose": 1,
+    # 使用 WsgiDAV 内置的内存锁管理器
+    "lock_storage": True,
+    # 禁用 WsgiDAV 的目录浏览器（纯 DAV 协议）
+    "dir_browser": {
+        "enable": False,
+    },
+}
+
+_wsgidav_app = WsgiDAVApp(_wsgidav_config)
+
+dav_app = WSGIMiddleware(_wsgidav_app, workers=10)
+"""ASGI 应用，挂载到 /dav 路径"""