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/application/ports/dedup_ports.rs

@@ -159,9 +159,6 @@ 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 in-memory content.
-    fn hash_bytes(&self, content: &[u8]) -> String;
-
     /// Calculate SHA-256 hash of a file (streaming).
     async fn hash_file(&self, path: &Path) -> Result<String, DomainError>;