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/external_mount_router.rs

@@ -78,6 +78,17 @@ impl MountRouter {
             ResolvedId::MountRoot { .. } | ResolvedId::MountChild { .. }
         )
     }
+
+    /// Path-based lookup for the protocol surfaces (WebDAV / NextCloud): does
+    /// `internal_path` descend into a mount within `drive_id`? Returns the mount
+    /// config plus the remainder relpath (empty when the path IS the mount root).
+    pub fn find_path(
+        &self,
+        drive_id: uuid::Uuid,
+        internal_path: &str,
+    ) -> Option<(Arc<MountConfig>, String)> {
+        self.registry.find_mount_for_path(drive_id, internal_path)
+    }
 }
 
 #[cfg(test)]