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/services/share_service.rs

@@ -479,6 +479,10 @@ mod tests {
         async fn get_parent_folder_id(&self, _path: &str) -> Result<String, DomainError> {
             unimplemented!()
         }
+
+        async fn get_blob_hash(&self, _file_id: &str) -> Result<String, DomainError> {
+            Ok(String::new())
+        }
     }
 
     #[async_trait]