docs: add Swagger annotations to 13 API handlers

Added @Summary, @Description, @Tags, @Param, @Success, @Failure,
@Router annotations to all major handler endpoints for OpenAPI/Swagger
auto-generation. Covers 86 annotations across:

- auth_handler.go (25): all auth endpoints
- user_handler.go (14): CRUD + roles + admin management
- device_handler.go (13): device CRUD + trust management
- role_handler.go (8): role CRUD + permissions
- custom_field_handler.go (7): field CRUD + user values
- permission_handler.go (7): permission CRUD + tree
- log_handler.go (3): login/operation logs
- captcha_handler.go (3): generate/verify
- stats_handler.go (2): dashboard + user stats
- avatar_handler.go (1): upload avatar
- totp_handler.go (1): totp status
- password_reset_handler.go (1): forgot password

Partially addresses P2: missing Swagger annotations
(PRODUCTION_GAP_ANALYSIS_2026-04-08)

internal/api/handler/stats_handler.go

@@ -18,6 +18,16 @@ func NewStatsHandler(statsService *service.StatsService) *StatsHandler {
 	return &StatsHandler{statsService: statsService}
 }
 
+// GetDashboard 获取仪表盘统计
+// @Summary 获取仪表盘统计
+// @Description 获取系统仪表盘统计数据（仅管理员）
+// @Tags 统计
+// @Produce json
+// @Security BearerAuth
+// @Success 200 {object} Response{data=service.DashboardStats} "仪表盘数据"
+// @Failure 403 {object} Response "无权限"
+// @Failure 500 {object} Response "服务器错误"
+// @Router /api/v1/admin/stats/dashboard [get]
 func (h *StatsHandler) GetDashboard(c *gin.Context) {
 	stats, err := h.statsService.GetDashboardStats(c.Request.Context())
 	if err != nil {
@@ -27,6 +37,16 @@ func (h *StatsHandler) GetDashboard(c *gin.Context) {
 	c.JSON(http.StatusOK, gin.H{"code": 0, "message": "success", "data": stats})
 }
 
+// GetUserStats 获取用户统计
+// @Summary 获取用户统计
+// @Description 获取用户统计数据（仅管理员）
+// @Tags 统计
+// @Produce json
+// @Security BearerAuth
+// @Success 200 {object} Response{data=service.UserStats} "用户统计数据"
+// @Failure 403 {object} Response "无权限"
+// @Failure 500 {object} Response "服务器错误"
+// @Router /api/v1/admin/stats/users [get]
 func (h *StatsHandler) GetUserStats(c *gin.Context) {
 	stats, err := h.statsService.GetUserStats(c.Request.Context())
 	if err != nil {