feat: add config-driven AI model tier routing

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/core/config.py

@@ -84,6 +84,27 @@ class Settings(BaseSettings):
     AI_MODEL_GEMINI: str = "gemini-2.5-flash"
     AI_MODEL_ANTHROPIC: str = "claude-sonnet-4-6"
 
+    # Model tier routing — maps action types to model tiers
+    AI_MODEL_TIERS: dict[str, str] = {
+        "fast": "claude-haiku-4-5-20251001",
+        "standard": "claude-sonnet-4-6-20250514",
+    }
+
+    ACTION_MODEL_MAP: dict[str, str] = {
+        "generate_full": "standard",
+        "generate_branch": "standard",
+        "modify_node": "fast",
+        "add_steps": "standard",
+        "quick_action": "fast",
+        "open_chat": "standard",
+        "variable_inference": "fast",
+    }
+
+    def get_model_for_action(self, action_type: str) -> str:
+        """Resolve an action type to a concrete model name via tier routing."""
+        tier = self.ACTION_MODEL_MAP.get(action_type, "standard")
+        return self.AI_MODEL_TIERS.get(tier, self.AI_MODEL_TIERS["standard"])
+
     # MCP (Model Context Protocol) integrations
     ENABLE_MCP_MICROSOFT_LEARN: bool = True