Fix Google image generation: model, response modality, data extraction

- NanoBanana Pro: gemini-2.0-flash-exp-image → gemini-3.1-flash-image-preview
- Add response_modalities=["IMAGE","TEXT"] so Gemini returns image data
- Replace part.as_image() with base64.b64decode(part.inline_data.data)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/services/image_generation_service.py

@@ -169,19 +169,26 @@ class ImageGenerationService:
         self, prompt: str, output_directory: str, model: str
     ) -> str:
         """Base method for Google image generation models."""
+        from google.genai import types
+
         client = genai.Client()
         response = await asyncio.to_thread(
             client.models.generate_content,
             model=model,
             contents=[prompt],
+            config=types.GenerateContentConfig(
+                response_modalities=["IMAGE", "TEXT"],
+            ),
         )
 
         image_path = None
         for part in response.candidates[0].content.parts:
             if part.inline_data is not None:
-                image = part.as_image()
+                image_data = base64.b64decode(part.inline_data.data)
                 image_path = os.path.join(output_directory, f"{uuid.uuid4()}.jpg")
-                image.save(image_path)
+                with open(image_path, "wb") as f:
+                    f.write(image_data)
+                break
 
         if not image_path:
             raise HTTPException(
@@ -201,9 +208,9 @@ class ImageGenerationService:
     async def generate_image_nanobanana_pro(
         self, prompt: str, output_directory: str
     ) -> str:
-        """Generate image using NanoBanana Pro (gemini-2.0-flash-exp-image)."""
+        """Generate image using NanoBanana Pro (gemini-3.1-flash-image-preview)."""
         return await self._generate_image_google(
-            prompt, output_directory, "gemini-2.0-flash-exp-image"
+            prompt, output_directory, "gemini-3.1-flash-image-preview"
         )
 
     async def get_image_from_pexels(self, prompt: str) -> str: