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.lock

@@ -442,6 +442,16 @@ dependencies = [
  "crossbeam-utils",
 ]
 
+[[package]]
+name = "crossbeam-deque"
+version = "0.8.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9dd111b7b7f7d55b72c0a6ae361660ee5853c9af73f70c3c2ef6858b950e2e51"
+dependencies = [
+ "crossbeam-epoch",
+ "crossbeam-utils",
+]
+
 [[package]]
 name = "crossbeam-epoch"
 version = "0.9.18"
@@ -1740,7 +1750,7 @@ checksum = "42f5e15c9953c5e4ccceeb2e7382a716482c34515315f7b03532b8b4e8393d2d"
 
 [[package]]
 name = "oxicloud"
-version = "0.3.5"
+version = "0.4.0"
 dependencies = [
  "anyhow",
  "argon2",
@@ -1768,6 +1778,7 @@ dependencies = [
  "moka",
  "quick-xml",
  "rand_core 0.6.4",
+ "rayon",
  "reqwest",
  "serde",
  "serde_json",
@@ -2170,6 +2181,26 @@ dependencies = [
  "getrandom 0.3.4",
 ]
 
+[[package]]
+name = "rayon"
+version = "1.11.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "368f01d005bf8fd9b1206fb6fa653e6c4a81ceb1466406b81792d87c5677a58f"
+dependencies = [
+ "either",
+ "rayon-core",
+]
+
+[[package]]
+name = "rayon-core"
+version = "1.13.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "22e18b0f0062d30d4230b2e85ff77fdfe4326feb054b9783a3460d8435c8ab91"
+dependencies = [
+ "crossbeam-deque",
+ "crossbeam-utils",
+]
+
 [[package]]
 name = "redox_syscall"
 version = "0.5.18"