feat: add hierarchical routing — groups can target other groups

RouteToConcrete() recursively resolves group chains until a concrete
model is reached, with cycle detection and max depth (10) guard.

Example: all-purpose -> fast-flow -> deepseek-v4-flash
The dashboard log shows the full chain: 'deepseek-v4-flash (hierarchical:
fast-flow (default (first target)) -> deepseek-v4-flash (default (first target)))'

internal/server/server.go

@@ -541,18 +541,20 @@ func (s *Server) handleChatCompletions(c *gin.Context) {
 		}
 	}
 
-	// Check if model is a group and route to a concrete model
+	// Resolve model groups to concrete models (hierarchical — groups can target groups)
 	modelGroup := ""
-	if s.modelRouter != nil && s.modelRouter.IsGroup(modelID) {
-		modelGroup = modelID
+	if s.modelRouter != nil {
 		userMessage := extractUserMessage(req.Messages)
-		decision, err := s.modelRouter.Route(c.Request.Context(), modelID, userMessage)
+		decision, err := s.modelRouter.RouteToConcrete(c.Request.Context(), modelID, userMessage)
 		if err != nil {
 			c.JSON(http.StatusInternalServerError, gin.H{"error": fmt.Sprintf("model routing failed: %v", err)})
 			return
 		}
+		if decision.SelectedModel != modelID {
+			modelGroup = modelID
+		}
 		modelID = decision.SelectedModel
-		log.Printf("[ROUTER] %s -> %s (%s: %s)", modelGroup, modelID, decision.Strategy, decision.Reason)
+		log.Printf("[ROUTER] %s (%s: %s)", modelID, decision.Strategy, decision.Reason)
 	}
 
 	// Convert ChatCompletionRequest to UnifiedRequest