refactor(router): split route registration into v1 package categorizations

- Created `internal/router/v1` subpackage
- Split routes into `admin.go`, `user.go`, `public.go`, and `custom.go`
- Cleaned up imports and helper registration functions from `router.go`
- Removed obsolete `internal/router/custom.go`

internal/router/v1/custom.go

@@ -0,0 +1,18 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+// Package v1 contains router registrations for API V1
+package v1
+
+import (
+	"github.com/Rain-kl/Wavelet/internal/apps/custom"
+	"github.com/gin-gonic/gin"
+)
+
+// RegisterCustomRoutes registers custom business routes to keep routing clean and stable.
+func RegisterCustomRoutes(apiV1Router *gin.RouterGroup) {
+	customRouter := apiV1Router.Group("/custom")
+	{
+		customRouter.GET("/hello", custom.Hello)
+	}
+}