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/dedup_ports.rs

@@ -15,7 +15,7 @@ use std::pin::Pin;
 /// Metadata of a stored blob in the dedup system.
 #[derive(Debug, Clone, Serialize)]
 pub struct BlobMetadataDto {
-    /// SHA-256 hash of the content.
+    /// BLAKE3 hash of the content.
     pub hash: String,
     /// Size in bytes.
     pub size: u64,
@@ -151,7 +151,7 @@ pub trait DedupPort: Send + Sync + 'static {
     /// Returns `true` if the blob was deleted (ref_count reached 0).
     async fn remove_reference(&self, hash: &str) -> Result<bool, DomainError>;
 
-    /// Calculate SHA-256 hash of a file (streaming).
+    /// Calculate BLAKE3 hash of a file (streaming).
     async fn hash_file(&self, path: &Path) -> Result<String, DomainError>;
 
     /// Get deduplication statistics.