feat: FlowStack 发布系统首个可运行版本 (gin + React)

- Go gin 后端：流水线/节点/连线/运行/日志/GitOps/插件 REST API，
  SQLite(GORM) 自动建表 + seed，模拟执行引擎 + SSE 实时日志推送，
  前端产物 go:embed 嵌入单二进制，一个端口同时服务 API 与页面
- React 19 前端：画布式流水线编辑器(React Flow + Tekton 任务库)、
  GitOps 部署、批量发布、发布记录等 14 个模块页面，
  tRPC 全部替换为 REST(TanStack Query)
- 首次启动生成随机密码管理员账号（日志 + data/initial-admin-password.txt）
- run.sh 一键构建启动（默认端口 9527，支持 start/stop/restart/build/dev）

backend/internal/handler/plugins.go

@@ -0,0 +1,87 @@
+package handler
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+
+	"flowstack/internal/database"
+	"flowstack/internal/model"
+)
+
+// availablePlugins 可安装插件市场（演示数据）
+var availablePlugins = []gin.H{
+	{"pluginId": "gitops-sync", "name": "GitOps Sync", "description": "Argo CD integration", "version": "1.3.0", "author": "OpsX Team", "category": "部署", "installs": 3420},
+	{"pluginId": "tekton-tasks", "name": "Tekton Tasks", "description": "Cloud-native CI tasks", "version": "2.1.0", "author": "CloudNative", "category": "构建", "installs": 5180},
+	{"pluginId": "dark-theme", "name": "Dark Theme", "description": "Dark UI theme", "version": "1.1.0", "author": "DesignLab", "category": "主题", "installs": 8920},
+	{"pluginId": "sonarqube", "name": "SonarQube Scanner", "description": "Code quality scan", "version": "3.2.0", "author": "CodeQuality", "category": "扫描", "installs": 2150},
+	{"pluginId": "vault-secrets", "name": "Vault Secrets", "description": "HashiCorp Vault integration", "version": "1.6.0", "author": "SecurityTeam", "category": "安全", "installs": 980},
+	{"pluginId": "matrix-build", "name": "Matrix Build", "description": "Multi-dimension parallel build", "version": "2.0.0", "author": "BuildMaster", "category": "构建", "installs": 1670},
+	{"pluginId": "slack-notify", "name": "Slack Notify", "description": "Slack notifications", "version": "1.3.0", "author": "NotifyHub", "category": "通知", "installs": 2890},
+}
+
+// ListPlugins GET /api/plugins
+func ListPlugins(c *gin.Context) {
+	var list []model.Plugin
+	database.DB.Order("created_at DESC").Find(&list)
+	c.JSON(http.StatusOK, list)
+}
+
+// AvailablePlugins GET /api/plugins/available
+func AvailablePlugins(c *gin.Context) {
+	c.JSON(http.StatusOK, availablePlugins)
+}
+
+// InstallPlugin POST /api/plugins/install
+func InstallPlugin(c *gin.Context) {
+	var body struct {
+		PluginID    string `json:"pluginId" binding:"required"`
+		Name        string `json:"name" binding:"required"`
+		Description string `json:"description"`
+		Version     string `json:"version" binding:"required"`
+		Author      string `json:"author"`
+		Category    string `json:"category"`
+		EntryPath   string `json:"entryPath"`
+	}
+	if err := c.ShouldBindJSON(&body); err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+		return
+	}
+	if body.Category == "" {
+		body.Category = "general"
+	}
+	p := model.Plugin{
+		PluginID: body.PluginID, Name: body.Name, Description: body.Description,
+		Version: body.Version, Author: body.Author, Category: body.Category, EntryPath: body.EntryPath,
+		Installed: true, Enabled: true, Config: map[string]any{},
+	}
+	if err := database.DB.Create(&p).Error; err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
+		return
+	}
+	c.JSON(http.StatusOK, p)
+}
+
+// TogglePlugin POST /api/plugins/:id/toggle {enabled}
+func TogglePlugin(c *gin.Context) {
+	var p model.Plugin
+	if err := database.DB.First(&p, c.Param("id")).Error; err != nil {
+		c.JSON(http.StatusNotFound, gin.H{"error": "plugin not found"})
+		return
+	}
+	var body struct {
+		Enabled bool `json:"enabled"`
+	}
+	if err := c.ShouldBindJSON(&body); err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+		return
+	}
+	database.DB.Model(&p).Update("enabled", body.Enabled)
+	c.JSON(http.StatusOK, gin.H{"success": true, "enabled": body.Enabled})
+}
+
+// UninstallPlugin DELETE /api/plugins/:id
+func UninstallPlugin(c *gin.Context) {
+	database.DB.Delete(&model.Plugin{}, c.Param("id"))
+	c.JSON(http.StatusOK, gin.H{"success": true})
+}