Fix Render: Use Python 3.12 and lightweight deps

Author: snigdhareddy482

render.yaml

@@ -1,8 +1,9 @@
 services:
   - type: web
     name: lumiclaim-backend
-    env: python
-    buildCommand: pip install -r requirements.txt
+    runtime: python
+    pythonVersion: "3.12.0"
+    buildCommand: pip install -r requirements-backend.txt
     startCommand: uvicorn backend.main:app --host 0.0.0.0 --port $PORT
     envVars:
       - key: GEMINI_API_KEY