Update Gemini model to gemini-1.5-flash

- Changed from deprecated gemini-pro to gemini-1.5-flash
- Fixes 404 error when generating recommendations

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

app/recommender.py

@@ -12,7 +12,7 @@ class BookRecommender:
         if not settings.gemini_api_key:
             raise ValueError("GEMINI_API_KEY not configured")
         genai.configure(api_key=settings.gemini_api_key)
-        self.model = genai.GenerativeModel('gemini-pro')
+        self.model = genai.GenerativeModel('gemini-1.5-flash')
 
     async def generate_recommendations(
         self,