feat: add AI generation service for network diagrams

Adds network_diagram_ai_service.py with generate_diagram() function that
calls the AI provider to convert plain-English network descriptions into
structured DiagramNode/DiagramEdge data. Registers the action in
ACTION_MODEL_MAP as a standard-tier route.

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

backend/app/core/config.py

@@ -105,6 +105,7 @@ class Settings(BaseSettings):
         "variable_inference": "fast",
         "kb_convert": "standard",
         "script_build": "standard",
+        "network_diagram_generate": "standard",
     }
 
     def get_model_for_action(self, action_type: str) -> str: