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>

pyproject.toml

@@ -33,6 +33,8 @@ dependencies = [
     "uvicorn>=0.38.0",
     "webauthn>=2.7.0",
     "whatthepatch>=1.0.6",
+    "wsgidav>=4.3.0",
+    "a2wsgi>=1.10.0",
 ]
 
 [project.optional-dependencies]