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/mod.rs

@@ -2,6 +2,5 @@ pub mod adapters;
 pub mod dtos;
 pub mod ports;
 pub mod services;
-pub mod transactions;
 
 // Re-exportaciones para facilitar el acceso a los principales puertos