fix: Updated requirements with test deps + proper pinning

backend/requirements.txt

@@ -1,6 +1,7 @@
-fastapi>=0.115.0
-uvicorn[standard]>=0.32.0
-httpx>=0.27.0
+fastapi>=0.104.0
+uvicorn[standard]>=0.24.0
+httpx>=0.25.0
+pydantic>=2.5.0
 python-dotenv>=1.0.0
-pydantic>=2.10.0
-python-multipart>=0.0.12
+pytest>=7.4.0
+pytest-asyncio>=0.21.0