fix(tests): resolve lazy loading errors in test_engine.py

Add await db.refresh() calls with relationship attributes before accessing
test_job.executions and execution.backups in async test context.

Fixes lazy loading errors in:
- test_execute_full_backup
- test_execute_incremental_without_full

backend/tests/test_engine.py

@@ -4,6 +4,7 @@ import tempfile
 import os
 from pathlib import Path
 from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy.orm import selectinload
 from app.models import Source, Job, JobExecution
 from backup.engine import BackupEngine
 
@@ -49,8 +50,14 @@ async def test_execute_full_backup(db: AsyncSession, test_job):
     assert execution.bytes_backed_up > 0
     assert execution.triggered_by == "manual"
     
+    # Refresh test_job to load executions relationship
+    await db.refresh(test_job, ["executions"])
+    
     # Verify backup was created
     assert len(test_job.executions) == 1
+    
+    # Refresh execution to load backups relationship
+    await db.refresh(test_job.executions[0], ["backups"])
     backup = test_job.executions[0].backups[0]
     assert backup.type == "full"
     assert backup.checksum is not None
@@ -67,6 +74,9 @@ async def test_execute_incremental_without_full(db: AsyncSession, test_job):
     
     # Should fall back to full backup
     assert execution.status == "success"
+    
+    # Refresh execution to load backups relationship
+    await db.refresh(execution, ["backups"])
     backup = execution.backups[0]
     assert backup.type == "full"
     assert backup.parent_backup_id is None