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/storage_ports.rs

@@ -55,7 +55,7 @@ pub trait FileReadPort: Send + Sync + 'static {
 
     /// Gets the content-addressable blob hash for a file (O(1) DB lookup).
     ///
-    /// Returns the SHA-256 hash stored in `storage.files.blob_hash`.
+    /// Returns the BLAKE3 hash stored in `storage.files.blob_hash`.
     /// Used for dedup reference tracking without loading file content.
     async fn get_blob_hash(&self, file_id: &str) -> Result<String, DomainError>;