perf: replace SHA-256 with BLAKE3 + add mimalloc global allocator

- Replace SHA-256 with BLAKE3 (~5x faster) for content-addressable hashing
  in dedup_service, file_handler, file_upload_service, chunked_upload_service
- Add mimalloc as global allocator for 10-30% throughput improvement
- sha2 crate retained only for PKCE (OAuth2 standard requirement)
- BLAKE3 produces 64-char hex hashes (same format), no DB schema changes needed

src/application/ports/chunked_upload_ports.rs

@@ -84,7 +84,7 @@ pub trait ChunkedUploadPort: Send + Sync + 'static {
 
     /// Assemble all chunks into the final file.
     ///
-    /// Returns `(assembled_file_path, filename, folder_id, content_type, total_size, sha256_hash)`.
+    /// Returns `(assembled_file_path, filename, folder_id, content_type, total_size, blake3_hash)`.
     /// The hash is computed during assembly (hash-on-write), eliminating a
     /// second sequential read of the assembled file.
     async fn complete_upload(