fix(webdav): fully working WebDAV RFC 4918 implementation

- Move WebDAV routes to top-level (out of /api nest) for proper path handling
- Add trailing slash routes and HEAD method support
- Refactor all 12 handlers to use Axum State extractor instead of req.extensions()
- Fix MOVE handler to support rename (same-folder move) via rename_file service
- Add Overwrite header support in MOVE/COPY operations
- Add extract_webdav_path() helper for consistent path parsing
- Add precondition_failed variant to AppError
- All 17 integration tests passing: OPTIONS, PROPFIND, MKCOL, PUT, GET, HEAD,
  PROPPATCH, COPY, MOVE, LOCK, DELETE (files and folders)

src/interfaces/errors.rs

@@ -78,6 +78,11 @@ impl AppError {
     pub fn unsupported_media_type(message: impl Into<String>) -> Self {
         Self::new(StatusCode::UNSUPPORTED_MEDIA_TYPE, message, "UnsupportedMediaType")
     }
+    
+    /// Create a 412 Precondition Failed error.
+    pub fn precondition_failed(message: impl Into<String>) -> Self {
+        Self::new(StatusCode::PRECONDITION_FAILED, message, "PreconditionFailed")
+    }
 }
 
 impl From<DomainError> for AppError {