Use correct Gemini model name: gemini-2.5-flash

- Changed from gemini-1.5-flash (deprecated) to gemini-2.5-flash
- This is the current available model in the Gemini API
- Fixes 404 model not found errors

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

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

app/recommender.py

@@ -12,7 +12,8 @@ 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-1.5-flash')
+        # Use the latest flash model available
+        self.model = genai.GenerativeModel('models/gemini-2.5-flash')
 
     async def generate_recommendations(
         self,