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/services/file_upload_service.rs

@@ -1,5 +1,5 @@
 use async_trait::async_trait;
-use sha2::{Digest, Sha256};
+
 use std::path::Path;
 use std::sync::Arc;
 
@@ -199,7 +199,7 @@ impl FileUploadUseCase for FileUploadService {
         tokio::fs::write(temp.path(), content)
             .await
             .map_err(|e| DomainError::internal_error("FileUpload", format!("write temp: {e}")))?;
-        let hash = hex::encode(Sha256::digest(content));
+        let hash = blake3::hash(content).to_hex().to_string();
 
         let file = self
             .file_write
@@ -228,7 +228,7 @@ impl FileUploadUseCase for FileUploadService {
         tokio::fs::write(temp.path(), content)
             .await
             .map_err(|e| DomainError::internal_error("FileUpload", format!("write temp: {e}")))?;
-        let hash = hex::encode(Sha256::digest(content));
+        let hash = blake3::hash(content).to_hex().to_string();
 
         self.update_file_streaming(
             path,