Use gemini-2.0-flash model

Gemini 1.5 models are not available through the API.
Use gemini-2.0-flash for fast, capable responses.

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

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

Author: robtaylor

chat-backend/main.py

@@ -26,7 +26,7 @@
 GCP_LOCATION = os.getenv("GCP_LOCATION", "us-central1")
 GEMINI_API_KEY = os.getenv("GEMINI_API_KEY", "")
 EMBEDDING_MODEL = "text-embedding-005"
-LLM_MODEL = "gemini-1.5-flash-latest"
+LLM_MODEL = "gemini-2.0-flash"
 
 # Allowed origins for CORS
 ALLOWED_ORIGINS = [