feat(retention): add count-based and days-based retention policies

backend/backup/engine.py

@@ -8,6 +8,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy import select
 from app.models import Job, JobExecution, Backup
 from backup.adapters import get_adapter
+from backup.retention import RetentionPolicy
 
 class BackupEngine:
     def __init__(self, db: AsyncSession):
@@ -109,6 +110,13 @@ class BackupEngine:
                 execution.bytes_processed = total_processed
                 execution.bytes_backed_up = total_backed_up
                 
+                # Apply retention policy
+                retention = RetentionPolicy(self.db)
+                keep_count = getattr(job, 'retention_count', None)
+                keep_days = getattr(job, 'retention_days', None)
+                if keep_count or keep_days:
+                    await retention.apply_retention_for_job(job_id, keep_count, keep_days)
+                
             finally:
                 await adapter.disconnect()