perf: fix issues #1,#2,#3,#13 from performance audit

- #1  image_transcode: dedicated rayon pool + moka cache (no tokio blocking)
- #2  ltree materialized paths: eliminate N+1 folder/file queries
- #3  compute_content_hash: read blob_hash column instead of loading file into RAM
- #13 event delegation + DocumentFragment: ~15 delegated listeners replace ~19k per-item listeners

Backend: 82/82 tests pass, cargo check clean.
Frontend: ui.js and app.js syntax-validated via Node.js.

src/infrastructure/services/dedup_service.rs

@@ -810,10 +810,6 @@ impl DedupPort for DedupService {
         self.remove_reference(hash).await
     }
 
-    fn hash_bytes(&self, content: &[u8]) -> String {
-        DedupService::hash_bytes(content)
-    }
-
     async fn hash_file(&self, path: &Path) -> Result<String, DomainError> {
         DedupService::hash_file(path)
             .await