Two fixes in dedup_service.rs:
1. Rename error path (L234): std::fs::remove_file → tokio::fs::remove_file
Restructured from map_err closure to match block since .await
cannot be used inside a sync closure.
2. Integrity verify (L691): fused blocking .exists() + async metadata()
into a single fs::metadata().await call. Eliminates one stat()
syscall per blob AND removes the only remaining blocking I/O from
the verify_integrity hot loop (buffer_unordered × VERIFY_CONCURRENCY).
Dionisio
962468a1ce