perf: Arc<AppState>, streaming PROPFIND, spawn_blocking SHA-256

- Issue #4: Wrap AppState in Arc — eliminates 42 Arc::clone + 16 String::clone per request
- Issue #2: Reject Depth:infinity with 403 + streaming XML with paginated DB queries
- Issue #5: Move chunked upload assembly (SHA-256 hash-on-write) to spawn_blocking
- Remove ~270 lines dead code from di.rs (unused builders, Default impl, stubs)
- Clean up unused tokio imports in chunked_upload_service.rs

src/application/services/file_retrieval_service.rs

@@ -317,4 +317,14 @@ impl FileRetrievalUseCase for FileRetrievalService {
         let files = self.file_read.list_files_in_subtree(folder_id).await?;
         Ok(files.into_iter().map(FileDto::from).collect())
     }
+
+    async fn list_files_batch(
+        &self,
+        folder_id: Option<&str>,
+        offset: i64,
+        limit: i64,
+    ) -> Result<Vec<FileDto>, DomainError> {
+        let files = self.file_read.list_files_batch(folder_id, offset, limit).await?;
+        Ok(files.into_iter().map(FileDto::from).collect())
+    }
 }