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.

Cargo.toml

@@ -48,6 +48,7 @@ http-body-util = "0.1.3"
 reqwest = { version = "0.12", default-features = false, features = ["json", "rustls-tls-webpki-roots"] }
 base64 = "0.22.1"
 fs2 = "0.4"
+rayon = "1.10"
 
 [features]
 default = []