fix: import AVAILABLE_MODELS from correct module to fix deploy crash

AVAILABLE_MODELS is defined in services/shared.py, not services/ai_service.py.
The incorrect import caused an ImportError on startup.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ashokDevs

backend/app.py

@@ -17,7 +17,8 @@
 
 import tasks
 from services import db as database
-from services.ai_service import AIService, AVAILABLE_MODELS, DEFAULT_MODEL
+from services.ai_service import AIService, DEFAULT_MODEL
+from services.shared import AVAILABLE_MODELS
 from services.github_service import GitHubService
 from services.groq_service import GroqService
 from services.redis_service import (