Revert to google.generativeai - new google.genai has different API

- Revert import back to google.generativeai (still works despite deprecation warning)
- Keep google-generativeai in requirements.txt
- The new google.genai package requires significant API changes (Client class instead of GenerativeModel)
- Will do proper migration later when we can test the new API thoroughly

requirements.txt

@@ -1,7 +1,7 @@
 fastapi
 uvicorn[standard]
 python-multipart
-google-genai>=1.38.0
+google-generativeai
 python-dotenv
 numpy
 opencv-python-headless

src/core.py

@@ -14,7 +14,7 @@ import base64
 import cv2
 import numpy as np
 from PIL import Image
-import google.genai as genai
+import google.generativeai as genai
 
 log = logging.getLogger(__name__)