Add database transaction support with atomic operations

- Create transaction.py with @transactional decorator, atomic() context manager
- Add TransactionPropagation modes: REQUIRED, REQUIRES_NEW, NESTED
- Add savepoint support for nested transactions with partial rollback
- Update connection.py with TransactionManager, get_transactional_session
- Update service.py with bulk operations (bulk_mark_downloaded, bulk_delete)
- Wrap QueueRepository.save_item() and clear_all() in atomic transactions
- Add comprehensive tests (66 transaction tests, 90% coverage)
- All 1090 tests passing

docs/CHANGELOG.md

@@ -71,6 +71,23 @@ This changelog follows [Keep a Changelog](https://keepachangelog.com/) principle
 
 _Changes that are in development but not yet released._
 
+### Added
+
+-   Database transaction support with `@transactional` decorator and `atomic()` context manager
+-   Transaction propagation modes (REQUIRED, REQUIRES_NEW, NESTED) for fine-grained control
+-   Savepoint support for nested transactions with partial rollback capability
+-   `TransactionManager` helper class for manual transaction control
+-   Bulk operations: `bulk_mark_downloaded`, `bulk_delete`, `clear_all` for batch processing
+-   `rotate_session` atomic operation for secure session rotation
+-   Transaction utilities: `is_session_in_transaction`, `get_session_transaction_depth`
+-   `get_transactional_session` for sessions without auto-commit
+
+### Changed
+
+-   `QueueRepository.save_item()` now uses atomic transactions for data consistency
+-   `QueueRepository.clear_all()` now uses atomic transactions for all-or-nothing behavior
+-   Service layer documentation updated to reflect transaction-aware design
+
 ### Fixed
 
 -   Scan status indicator now correctly shows running state after page reload during active scan