refactor(layout): consolidate backend codebase into backend/ package and clean root directory

- Moved cmd/, core/, plugins/, pkg/, downstream/, and main.go into backend/ directory
- Batch updated all Go source files to import github.com/Rain-kl/Wavelet/backend/...
- Updated Makefile, scripts/swagger.sh, architecture guards, and platform skills
- Passed all quality gates (100% tests, 0 lint issues, clean build)

backend/plugins/domain/system/plugin.go

@@ -0,0 +1,72 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+// Package system provides core system health probes, public config endpoints, and static frontend assets dispatch plugin for Cordis.
+package system
+
+import (
+	"net/http"
+
+	"github.com/Rain-kl/Wavelet/backend/core"
+	"github.com/Rain-kl/Wavelet/backend/pkg/config"
+	"github.com/Rain-kl/Wavelet/backend/pkg/response"
+	database "github.com/Rain-kl/Wavelet/backend/plugins/infra/database"
+	"github.com/gin-gonic/gin"
+)
+
+// Plugin implements core.Plugin to provide system-level basic routes.
+type Plugin struct{}
+
+// New creates a new system domain plugin.
+func New() *Plugin {
+	return &Plugin{}
+}
+
+// Name returns the unique identifier for the system domain plugin.
+func (p *Plugin) Name() string {
+	return "system"
+}
+
+// Manifest returns the plugin metadata.
+func (p *Plugin) Manifest() core.Manifest {
+	return core.Manifest{
+		Name:        "system",
+		Version:     "1.0.0",
+		Description: "System health check, public config, and assets domain plugin",
+		Author:      "Wavelet Team",
+	}
+}
+
+// Apply registers system routes.
+func (p *Plugin) Apply(ctx *core.Context) error {
+	// 1. Health check
+	ctx.Router().GET("/healthz", func(c *gin.Context) {
+		c.JSON(http.StatusOK, gin.H{"status": "ok"})
+	})
+	ctx.Router().GET("/api/healthz", func(c *gin.Context) {
+		c.JSON(http.StatusOK, gin.H{"status": "ok"})
+	})
+
+	// 2. Public config
+	ctx.Router().GET("/api/v1/config/public", func(c *gin.Context) {
+		type configItem struct {
+			Key   string `json:"key"`
+			Value string `json:"value"`
+		}
+		var configs []configItem
+		_ = database.DB(c.Request.Context()).Table("w_system_configs").Where("visibility = ?", "visible").Find(&configs).Error
+		c.JSON(http.StatusOK, response.OK(gin.H{
+			"configs": configs,
+			"app": gin.H{
+				"name": config.Config.App.AppName,
+			},
+		}))
+	})
+
+	// 3. Custom injection
+	ctx.Router().GET("/custom", func(c *gin.Context) {
+		c.JSON(http.StatusOK, response.OK(gin.H{"custom": true}))
+	})
+
+	return nil
+}