Fix: Unpin all dependencies to support Python 3.13 default in Render

backend/requirements.txt

@@ -1,14 +1,14 @@
-fastapi==0.110.0
-uvicorn==0.29.0
-python-dotenv==1.0.1
-pydantic==2.7.0
-requests==2.31.0
-python-multipart==0.0.9
+fastapi>=0.110.0
+uvicorn>=0.30.0
+python-dotenv>=1.0.1
+pydantic>=2.10.0
+requests>=2.32.0
+python-multipart>=0.0.9
 # AI / LangGraph
-langgraph==0.0.30
-langchain-core==0.1.40
-langchain-openai==0.1.1
+langgraph>=0.2.0
+langchain-core>=0.3.0
+langchain-openai>=0.2.0
 # Media Services
-assemblyai==0.26.0
-elevenlabs==1.0.0
-google-generativeai==0.5.0
+assemblyai>=0.33.0
+elevenlabs>=1.0.0
+google-generativeai>=0.8.0