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

data/config_backups/config_backup_20251225_134617.json

@@ -0,0 +1,23 @@
+{
+  "name": "Aniworld",
+  "data_dir": "data",
+  "scheduler": {
+    "enabled": true,
+    "interval_minutes": 60
+  },
+  "logging": {
+    "level": "INFO",
+    "file": null,
+    "max_bytes": null,
+    "backup_count": 3
+  },
+  "backup": {
+    "enabled": false,
+    "path": "data/backups",
+    "keep_days": 30
+  },
+  "other": {
+    "master_password_hash": "$pbkdf2-sha256$29000$gvDe27t3TilFiHHOuZeSMg$zEPyA6XcqVVTz7raeXZnMtGt/Q5k8ZCl204K0hx5z0w"
+  },
+  "version": "1.0.0"
+}

data/config_backups/config_backup_20251225_134748.json

@@ -0,0 +1,23 @@
+{
+  "name": "Aniworld",
+  "data_dir": "data",
+  "scheduler": {
+    "enabled": true,
+    "interval_minutes": 60
+  },
+  "logging": {
+    "level": "INFO",
+    "file": null,
+    "max_bytes": null,
+    "backup_count": 3
+  },
+  "backup": {
+    "enabled": false,
+    "path": "data/backups",
+    "keep_days": 30
+  },
+  "other": {
+    "master_password_hash": "$pbkdf2-sha256$29000$1pqTMkaoFSLEWKsVAmBsDQ$DHVcHMFFYJxzYmc.7LnDru61mYtMv9PMoxPgfuKed/c"
+  },
+  "version": "1.0.0"
+}

data/config_backups/config_backup_20251225_180408.json

@@ -0,0 +1,23 @@
+{
+  "name": "Aniworld",
+  "data_dir": "data",
+  "scheduler": {
+    "enabled": true,
+    "interval_minutes": 60
+  },
+  "logging": {
+    "level": "INFO",
+    "file": null,
+    "max_bytes": null,
+    "backup_count": 3
+  },
+  "backup": {
+    "enabled": false,
+    "path": "data/backups",
+    "keep_days": 30
+  },
+  "other": {
+    "master_password_hash": "$pbkdf2-sha256$29000$ndM6hxDC.F8LYUxJCSGEEA$UHGXMaEruWVgpRp8JI/siGETH8gOb20svhjy9plb0Wo"
+  },
+  "version": "1.0.0"
+}