perf: eliminate Vec<u8> buffer paths — all uploads now stream to disk

Issue #4 (HIGH): save_file(Vec<u8>) and update_file_content(Vec<u8>)
accepted up to 10 MB of contiguous memory per request. While the main
upload paths already used streaming, the WebDAV compat methods
(create_file, update_file) and the empty-file handler still used the
buffered path, creating a .to_vec() copy.

Changes:
- FileWritePort trait: remove save_file(Vec<u8>) and
  update_file_content(Vec<u8>) — only streaming variants remain
- FileUploadUseCase trait: remove upload_file(Vec<u8>)
- file_upload_service.rs: create_file() and update_file() now spool
  &[u8] to NamedTempFile + Sha256::digest, then delegate to streaming
  path (save_file_from_temp / update_file_streaming)
- file_handler.rs: empty file uploads use upload_file_streaming with
- FileBlobWriteRepository: remove save_file and update_file_content impls
- StubFileWritePort, StubFileUploadUseCase, MockFileRepository: remove
  corresponding dead method impls

Impact: impossible to accidentally use a buffered upload path. All
content goes through streaming with ~256 KB peak RAM. -166 LOC.

src/interfaces/api/handlers/file_handler.rs

@@ -167,11 +167,18 @@ impl FileHandler {
                     ));
                 }
 
-                // Empty file — use in-memory path
+                // Empty file — use streaming path with the (empty) temp file
                 if total_size == 0 {
-                    let _ = tokio::fs::remove_file(&temp_path).await;
+                    let hash = hex::encode(hasher.finalize());
                     return upload_service
-                        .upload_file(filename, folder_id, content_type, vec![])
+                        .upload_file_streaming(
+                            filename,
+                            folder_id,
+                            content_type,
+                            &temp_path,
+                            0,
+                            Some(hash),
+                        )
                         .await
                         .map_err(Self::domain_error_response);
                 }