fix(thumbnails): resolve blob path instead of logical path for thumbnail generation

Since the blob storage migration (3c7c16f), thumbnail generation failed
with "No such file or directory" because the handler constructed logical
file paths that don't exist on disk. Resolve the actual blob path via
get_blob_hash() + dedup_service.blob_path() in both upload and get
thumbnail handlers. Add regression tests.

src/infrastructure/services/mod.rs

@@ -11,6 +11,8 @@ pub mod password_hasher;
 pub mod path_resolver_service;
 pub mod path_service;
 pub mod thumbnail_service;
+#[cfg(test)]
+mod thumbnail_service_test;
 pub mod trash_cleanup_service;
 pub mod webdav_lock_service;
 pub mod wopi_discovery_service;