feat(mounts): P3 — WebDAV/NextCloud path resolution + browse/download

Mounts are now browsable and downloadable over both WebDAV surfaces
(/webdav/ and NextCloud /remote.php/dav) and NextCloud clients. The work
funnels through the path-based service methods all three surfaces already use,
so both WebDAV handlers gain mount support with no handler changes.

- get_folder_by_path / get_file_by_path resolve a path that descends past a
  mount root to a synthetic ext: DTO (the mount root itself stays a real row)
- list_folders_paginated_with_perms / list_files_batch_with_perms branch to the
  provider, so PROPFIND Depth:1 enumerates mount directory contents in both
  WebDAV handlers
- get_file_stream / get_file_range_stream branch ext: file ids to the provider,
  so WebDAV GET streams mount content (Pin<Box<dyn Stream>> re-boxed)
- router injected into FileRetrievalService; new MountRouter::find_path delegates
  to the registry's (drive_id, mount_path) index
- WebDAV mkdir/delete/move/rename by path work via path→ext:id→the P2 service
  methods (no extra wiring)

Fixes a leading-slash normalization bug in the registry path index: materialized
folder paths arrive both as `Personal/Media` and `/Personal/Media`; keys and
lookups now normalize the leading slash (would have broken real WebDAV paths).

Known follow-up: WebDAV PUT (upload/update by path) still ingests to the CAS;
streaming a WebDAV PUT straight to the provider needs a pre-ingest branch in the
two WebDAV PUT handlers (mirrors the REST upload branch).

Integration test: get_folder_by_path/get_file_by_path resolution, PROPFIND
Depth:1 folder+file listing, and content streaming on a real mount + Postgres.

src/common/di.rs

@@ -531,12 +531,15 @@ impl AppServiceFactory {
         // bridge (which looks file metadata up by id) can be wired into the
         // dispatcher they receive. It depends only on repos + core, never on
         // the upload service, so the reorder is safe.
-        let file_retrieval_service = Arc::new(FileRetrievalService::new_with_cache(
-            repos.file_read_repository.clone(),
-            core.file_content_cache.clone(),
-            core.image_transcode_service.clone(),
-            authz.clone(),
-        ));
+        let file_retrieval_service = Arc::new(
+            FileRetrievalService::new_with_cache(
+                repos.file_read_repository.clone(),
+                core.file_content_cache.clone(),
+                core.image_transcode_service.clone(),
+                authz.clone(),
+            )
+            .with_mount_router(mount_router.clone()),
+        );
 
         // Effective lifecycle dispatcher: the core hooks (thumbnails, metadata)
         // plus, when the plugins feature is enabled, the WASM plugin bridge.