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/application/services/mount_registry.rs

@@ -90,6 +90,11 @@ impl MountRegistry {
         internal_path: &str,
     ) -> Option<(Arc<MountConfig>, String)> {
         let index = self.inner.load();
+        // Normalize away a leading slash: materialized folder paths arrive both
+        // as `Personal/Media` (raw `folders.path`) and `/Personal/Media`
+        // (FolderDto / WebDAV internal paths). The index keys are stored without
+        // a leading slash (see `reload`).
+        let internal_path = internal_path.trim_start_matches('/');
         // Walk ancestor paths from the full path up to the root, longest first,
         // so the deepest matching mount wins.
         let mut candidate = internal_path;
@@ -144,7 +149,15 @@ impl MountRegistry {
                     continue;
                 }
             };
-            by_path.insert((rec.drive_id, rec.mount_path.clone()), rec.mount_folder_id);
+            // Store the path key without a leading slash so lookups normalize
+            // consistently (see `find_mount_for_path`).
+            by_path.insert(
+                (
+                    rec.drive_id,
+                    rec.mount_path.trim_start_matches('/').to_string(),
+                ),
+                rec.mount_folder_id,
+            );
             by_folder.insert(
                 rec.mount_folder_id,
                 Arc::new(MountConfig {