Add ref_count management for deletions with atomic operations and error handling

backend/requirements.txt

@@ -9,3 +9,10 @@ pydantic==2.5.3
 pydantic-settings==2.1.0
 python-jose[cryptography]==3.3.0
 passlib[bcrypt]==1.7.4
+
+# Test dependencies
+pytest>=7.4.0
+pytest-asyncio>=0.21.0
+pytest-cov>=4.1.0
+httpx>=0.25.0
+moto[s3]>=4.2.0