perf(webdav): single UNION ALL query for path resolution

Replace the double-query pattern (get_folder_by_path + get_file_by_path)
across PROPFIND, HEAD, DELETE, MOVE, and COPY handlers with a single
UNION ALL query via PathResolverService.

PG Append node short-circuits on LIMIT 1: if the folder branch matches,
the file branch is never executed. Cuts WebDAV path resolution from
2 round-trips to 1 per request.

Also adds an exists() method using EXISTS subqueries for the Overwrite
header checks in MOVE/COPY (avoids constructing full DTOs).

Legacy double-query fallback retained when PathResolver is unavailable.

src/common/di.rs

@@ -542,6 +542,7 @@ impl AppServiceFactory {
             wopi_discovery_service: None,
             device_auth_service: None,
             app_password_service: None,
+            path_resolver: None,
         };
 
         // 9b. Wire admin settings service when auth is available
@@ -651,6 +652,13 @@ impl AppServiceFactory {
             }
         }
 
+        // 9e. Wire PathResolver for single-query WebDAV path resolution
+        {
+            use crate::infrastructure::services::path_resolver_service::PathResolverService;
+            app_state.path_resolver = Some(Arc::new(PathResolverService::new(pool.clone())));
+            tracing::info!("PathResolver service initialized");
+        }
+
         // 10. Wire CalDAV/CardDAV services
         {
             // CalDAV
@@ -846,6 +854,8 @@ pub struct AppState {
         Option<Arc<crate::application::services::device_auth_service::DeviceAuthService>>,
     pub app_password_service:
         Option<Arc<crate::application::services::app_password_service::AppPasswordService>>,
+    pub path_resolver:
+        Option<Arc<crate::infrastructure::services::path_resolver_service::PathResolverService>>,
 }
 
 // All AppState construction is done via struct literal in build_app_state().