perf: optimize hot paths — batch concurrency, pagination, sorting, transcoding, folder ops

- batch_operations.rs: replace join_all with buffer_unordered, Arc<str> for shared IDs, remove redundant clones and dead Semaphore
- folder_db_repository.rs: use COUNT(*) OVER() for single-query pagination; UPDATE RETURNING for rename/move (eliminates extra SELECTs)
- folder_service.rs: remove StorageTransaction wrapper from rename/move — direct repo call (4→2 and 5→3 queries)
- search_service.rs: replace sort_by(to_lowercase) with sort_by_cached_key (N vs 2·N·log₂N allocations)
- image_transcode_service.rs: dynamic rayon pool sizing via available_parallelism() instead of hardcoded 2 threads
- Remove dead transactions module (zero consumers after folder_service refactor)

src/application/services/search_service.rs

@@ -1,4 +1,5 @@
 use async_trait::async_trait;
+use std::cmp::Reverse;
 use std::sync::Arc;
 use std::time::{Duration, Instant};
 
@@ -323,15 +324,13 @@ impl SearchUseCase for SearchService {
                 .map(|f| Self::enrich_folder(f, query))
                 .collect();
 
-            // Sort folders
+            // Sort folders (cached_key avoids O(N log N) temporary String allocations)
             match criteria.sort_by.as_str() {
                 "name" => {
-                    enriched_folders
-                        .sort_by(|a, b| a.name.to_lowercase().cmp(&b.name.to_lowercase()));
+                    enriched_folders.sort_by_cached_key(|f| f.name.to_lowercase());
                 }
                 "name_desc" => {
-                    enriched_folders
-                        .sort_by(|a, b| b.name.to_lowercase().cmp(&a.name.to_lowercase()));
+                    enriched_folders.sort_by_cached_key(|f| Reverse(f.name.to_lowercase()));
                 }
                 "date" => {
                     enriched_folders.sort_by(|a, b| a.modified_at.cmp(&b.modified_at));
@@ -411,13 +410,13 @@ impl SearchUseCase for SearchService {
             .map(|f| Self::enrich_folder(f, query))
             .collect();
 
-        // ── Sort folders (files already sorted by SQL ORDER BY) ──
+        // ── Sort folders (cached_key avoids O(N log N) temporary String allocations) ──
         match criteria.sort_by.as_str() {
             "name" => {
-                enriched_folders.sort_by(|a, b| a.name.to_lowercase().cmp(&b.name.to_lowercase()));
+                enriched_folders.sort_by_cached_key(|f| f.name.to_lowercase());
             }
             "name_desc" => {
-                enriched_folders.sort_by(|a, b| b.name.to_lowercase().cmp(&a.name.to_lowercase()));
+                enriched_folders.sort_by_cached_key(|f| Reverse(f.name.to_lowercase()));
             }
             "date" => {
                 enriched_folders.sort_by(|a, b| a.modified_at.cmp(&b.modified_at));