Rename app_kernel.py to app.py and update imports

Renamed src/backend/app_kernel.py to src/backend/app.py. Updated import paths to reference 'af' modules instead of 'v3' and 'messages_af' instead of 'messages_kernel' to reflect new module structure.

Author: Fr4nc3

src/backend/app_kernel.py renamed to src/backend/app.py

@@ -6,20 +6,20 @@
 
 from azure.monitor.opentelemetry import configure_azure_monitor
 from common.config.app_config import config
-from common.models.messages_kernel import UserLanguage
+from common.models.messages_af import UserLanguage
 
 # FastAPI imports
 from fastapi import FastAPI, Request
 from fastapi.middleware.cors import CORSMiddleware
 
 # Local imports
 from middleware.health_check import HealthCheckMiddleware
-from v3.api.router import app_v3
+from af.api.router import app_v3
 
 # Azure monitoring
 
 # Semantic Kernel imports
-from v3.config.agent_registry import agent_registry
+from af.config.agent_registry import agent_registry
 
 
 @asynccontextmanager