feat: Phase 2-5 监控中心全栈实现 — 指标浏览器/SNMP/站点拨测/中间件

Phase 2 指标监控:
- 修复 /metrics 路由(原指向 AlertManager → 现指向 MetricsExplorer)
- 新建 MetricsHandler (4 API): targets/list/query/names
- 新建 MetricsExplorer 前端(170行): 目标选择器+指标分类树+多指标叠加SVG图表

Phase 3 SNMP设备:
- 新建 SNMPHandler (9 API): devices CRUD/templates/stats
- SNMPMonitor 接入真实API(428行): 设备表格+添加对话框+删除

Phase 4 站点拨测:
- 新建 SiteHandler (8 API): tasks CRUD/results/stats
- SiteMonitor 接入真实API(542行): 任务表+添加对话框+拨测类型网格
- 新增 siteApi 到 api.ts

Phase 5 中间件采集:
- Agent 新增 collector/middleware.go: Redis INFO + MySQL handshake 采集
- Agent CLI 新增 -redis/-redis-auth/-mysql/-mysql-user/-mysql-pass
- MiddlewareMonitor 接入真实API(301行): 实例卡片+指标面板

基础设施:
- AutoMigrate 新增 23 张监控表
- 前后端容器构建部署验证通过

backend/internal/api/handlers/snmp.go

@@ -0,0 +1,143 @@
+package handlers
+
+import (
+	"ezops/internal/models"
+	"net/http"
+	"strconv"
+
+	"github.com/gin-gonic/gin"
+	"gorm.io/gorm"
+)
+
+// SNMPHandler SNMP 设备管理 handler // Logan.Li
+type SNMPHandler struct {
+	db *gorm.DB
+}
+
+// NewSNMPHandler 创建 SNMP handler
+func NewSNMPHandler(db *gorm.DB) *SNMPHandler {
+	return &SNMPHandler{db: db}
+}
+
+// ========== SNMP Device CRUD ==========
+
+// ListDevices 列出 SNMP 设备
+// GET /api/v1/snmp/devices?page=1&page_size=20&keyword=
+func (h *SNMPHandler) ListDevices(c *gin.Context) {
+	var devices []models.SNMPDevice
+	q := h.db.Model(&models.SNMPDevice{})
+
+	if kw := c.Query("keyword"); kw != "" {
+		q = q.Where("name LIKE ? OR ip LIKE ?", "%"+kw+"%", "%"+kw+"%")
+	}
+
+	page, _ := strconv.Atoi(c.DefaultQuery("page", "1"))
+	pageSize, _ := strconv.Atoi(c.DefaultQuery("page_size", "20"))
+	var total int64
+	q.Count(&total)
+	q.Offset((page - 1) * pageSize).Limit(pageSize).Order("name ASC").Find(&devices)
+
+	if devices == nil {
+		devices = []models.SNMPDevice{}
+	}
+	c.JSON(http.StatusOK, models.Response{Code: 200, Data: gin.H{"list": devices, "total": total}})
+}
+
+// CreateDevice 创建 SNMP 设备
+// POST /api/v1/snmp/devices
+func (h *SNMPHandler) CreateDevice(c *gin.Context) {
+	var device models.SNMPDevice
+	if err := c.ShouldBindJSON(&device); err != nil {
+		c.JSON(http.StatusBadRequest, models.Response{Code: 400, Message: err.Error()})
+		return
+	}
+	if err := h.db.Create(&device).Error; err != nil {
+		c.JSON(http.StatusInternalServerError, models.Response{Code: 500, Message: err.Error()})
+		return
+	}
+	c.JSON(http.StatusOK, models.Response{Code: 200, Data: device, Message: "创建成功"})
+}
+
+// UpdateDevice 更新 SNMP 设备
+// PUT /api/v1/snmp/devices/:id
+func (h *SNMPHandler) UpdateDevice(c *gin.Context) {
+	id := c.Param("id")
+	var device models.SNMPDevice
+	if err := h.db.First(&device, id).Error; err != nil {
+		c.JSON(http.StatusNotFound, models.Response{Code: 404, Message: "设备不存在"})
+		return
+	}
+	if err := c.ShouldBindJSON(&device); err != nil {
+		c.JSON(http.StatusBadRequest, models.Response{Code: 400, Message: err.Error()})
+		return
+	}
+	h.db.Save(&device)
+	c.JSON(http.StatusOK, models.Response{Code: 200, Data: device, Message: "更新成功"})
+}
+
+// DeleteDevice 删除 SNMP 设备
+// DELETE /api/v1/snmp/devices/:id
+func (h *SNMPHandler) DeleteDevice(c *gin.Context) {
+	id := c.Param("id")
+	if err := h.db.Delete(&models.SNMPDevice{}, id).Error; err != nil {
+		c.JSON(http.StatusInternalServerError, models.Response{Code: 500, Message: err.Error()})
+		return
+	}
+	c.JSON(http.StatusOK, models.Response{Code: 200, Message: "删除成功"})
+}
+
+// BatchDeleteDevices 批量删除
+// DELETE /api/v1/snmp/devices/batch-delete
+func (h *SNMPHandler) BatchDeleteDevices(c *gin.Context) {
+	var req models.IDsRequest
+	if err := c.ShouldBindJSON(&req); err != nil {
+		c.JSON(http.StatusBadRequest, models.Response{Code: 400, Message: err.Error()})
+		return
+	}
+	h.db.Where("id IN ?", req.IDs).Delete(&models.SNMPDevice{})
+	c.JSON(http.StatusOK, models.Response{Code: 200, Message: "批量删除成功"})
+}
+
+// ========== SNMP Template CRUD ==========
+
+// ListTemplates 列出 OID 模板
+// GET /api/v1/snmp/templates
+func (h *SNMPHandler) ListTemplates(c *gin.Context) {
+	var templates []models.SNMPTemplate
+	h.db.Order("name ASC").Find(&templates)
+	if templates == nil {
+		templates = []models.SNMPTemplate{}
+	}
+	c.JSON(http.StatusOK, models.Response{Code: 200, Data: templates})
+}
+
+// CreateTemplate 创建 OID 模板
+// POST /api/v1/snmp/templates
+func (h *SNMPHandler) CreateTemplate(c *gin.Context) {
+	var template models.SNMPTemplate
+	if err := c.ShouldBindJSON(&template); err != nil {
+		c.JSON(http.StatusBadRequest, models.Response{Code: 400, Message: err.Error()})
+		return
+	}
+	if err := h.db.Create(&template).Error; err != nil {
+		c.JSON(http.StatusInternalServerError, models.Response{Code: 500, Message: err.Error()})
+		return
+	}
+	c.JSON(http.StatusOK, models.Response{Code: 200, Data: template, Message: "创建成功"})
+}
+
+// ========== SNMP Statistics ==========
+
+// GetSNMPStats 获取 SNMP 概览统计
+// GET /api/v1/snmp/stats
+func (h *SNMPHandler) GetSNMPStats(c *gin.Context) {
+	var total, online int64
+	h.db.Model(&models.SNMPDevice{}).Count(&total)
+	h.db.Model(&models.SNMPDevice{}).Where("status = ?", "online").Count(&online)
+
+	c.JSON(http.StatusOK, models.Response{Code: 200, Data: gin.H{
+		"total":  total,
+		"online": online,
+		"abnormal": total - online,
+	}})
+}