feat: 实现注册优惠码功能

- 支持创建/编辑/删除优惠码，设置赠送金额和使用限制
  - 注册页面实时验证优惠码并显示赠送金额
  - 支持 URL 参数自动填充 (?promo=CODE)
  - 添加优惠码验证接口速率限制
  - 使用数据库行锁防止并发超限
  - 新增后台优惠码管理页面，支持复制注册链接

backend/internal/handler/admin/promo_handler.go

@@ -0,0 +1,209 @@
+package admin
+
+import (
+	"strconv"
+	"strings"
+	"time"
+
+	"github.com/Wei-Shaw/sub2api/internal/handler/dto"
+	"github.com/Wei-Shaw/sub2api/internal/pkg/pagination"
+	"github.com/Wei-Shaw/sub2api/internal/pkg/response"
+	"github.com/Wei-Shaw/sub2api/internal/service"
+
+	"github.com/gin-gonic/gin"
+)
+
+// PromoHandler handles admin promo code management
+type PromoHandler struct {
+	promoService *service.PromoService
+}
+
+// NewPromoHandler creates a new admin promo handler
+func NewPromoHandler(promoService *service.PromoService) *PromoHandler {
+	return &PromoHandler{
+		promoService: promoService,
+	}
+}
+
+// CreatePromoCodeRequest represents create promo code request
+type CreatePromoCodeRequest struct {
+	Code        string  `json:"code"`                                    // 可选，为空则自动生成
+	BonusAmount float64 `json:"bonus_amount" binding:"required,min=0"`   // 赠送余额
+	MaxUses     int     `json:"max_uses" binding:"min=0"`                // 最大使用次数，0=无限
+	ExpiresAt   *int64  `json:"expires_at"`                              // 过期时间戳（秒）
+	Notes       string  `json:"notes"`                                   // 备注
+}
+
+// UpdatePromoCodeRequest represents update promo code request
+type UpdatePromoCodeRequest struct {
+	Code        *string  `json:"code"`
+	BonusAmount *float64 `json:"bonus_amount" binding:"omitempty,min=0"`
+	MaxUses     *int     `json:"max_uses" binding:"omitempty,min=0"`
+	Status      *string  `json:"status" binding:"omitempty,oneof=active disabled"`
+	ExpiresAt   *int64   `json:"expires_at"`
+	Notes       *string  `json:"notes"`
+}
+
+// List handles listing all promo codes with pagination
+// GET /api/v1/admin/promo-codes
+func (h *PromoHandler) List(c *gin.Context) {
+	page, pageSize := response.ParsePagination(c)
+	status := c.Query("status")
+	search := strings.TrimSpace(c.Query("search"))
+	if len(search) > 100 {
+		search = search[:100]
+	}
+
+	params := pagination.PaginationParams{
+		Page:     page,
+		PageSize: pageSize,
+	}
+
+	codes, paginationResult, err := h.promoService.List(c.Request.Context(), params, status, search)
+	if err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
+
+	out := make([]dto.PromoCode, 0, len(codes))
+	for i := range codes {
+		out = append(out, *dto.PromoCodeFromService(&codes[i]))
+	}
+	response.Paginated(c, out, paginationResult.Total, page, pageSize)
+}
+
+// GetByID handles getting a promo code by ID
+// GET /api/v1/admin/promo-codes/:id
+func (h *PromoHandler) GetByID(c *gin.Context) {
+	codeID, err := strconv.ParseInt(c.Param("id"), 10, 64)
+	if err != nil {
+		response.BadRequest(c, "Invalid promo code ID")
+		return
+	}
+
+	code, err := h.promoService.GetByID(c.Request.Context(), codeID)
+	if err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
+
+	response.Success(c, dto.PromoCodeFromService(code))
+}
+
+// Create handles creating a new promo code
+// POST /api/v1/admin/promo-codes
+func (h *PromoHandler) Create(c *gin.Context) {
+	var req CreatePromoCodeRequest
+	if err := c.ShouldBindJSON(&req); err != nil {
+		response.BadRequest(c, "Invalid request: "+err.Error())
+		return
+	}
+
+	input := &service.CreatePromoCodeInput{
+		Code:        req.Code,
+		BonusAmount: req.BonusAmount,
+		MaxUses:     req.MaxUses,
+		Notes:       req.Notes,
+	}
+
+	if req.ExpiresAt != nil {
+		t := time.Unix(*req.ExpiresAt, 0)
+		input.ExpiresAt = &t
+	}
+
+	code, err := h.promoService.Create(c.Request.Context(), input)
+	if err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
+
+	response.Success(c, dto.PromoCodeFromService(code))
+}
+
+// Update handles updating a promo code
+// PUT /api/v1/admin/promo-codes/:id
+func (h *PromoHandler) Update(c *gin.Context) {
+	codeID, err := strconv.ParseInt(c.Param("id"), 10, 64)
+	if err != nil {
+		response.BadRequest(c, "Invalid promo code ID")
+		return
+	}
+
+	var req UpdatePromoCodeRequest
+	if err := c.ShouldBindJSON(&req); err != nil {
+		response.BadRequest(c, "Invalid request: "+err.Error())
+		return
+	}
+
+	input := &service.UpdatePromoCodeInput{
+		Code:        req.Code,
+		BonusAmount: req.BonusAmount,
+		MaxUses:     req.MaxUses,
+		Status:      req.Status,
+		Notes:       req.Notes,
+	}
+
+	if req.ExpiresAt != nil {
+		if *req.ExpiresAt == 0 {
+			// 0 表示清除过期时间
+			input.ExpiresAt = nil
+		} else {
+			t := time.Unix(*req.ExpiresAt, 0)
+			input.ExpiresAt = &t
+		}
+	}
+
+	code, err := h.promoService.Update(c.Request.Context(), codeID, input)
+	if err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
+
+	response.Success(c, dto.PromoCodeFromService(code))
+}
+
+// Delete handles deleting a promo code
+// DELETE /api/v1/admin/promo-codes/:id
+func (h *PromoHandler) Delete(c *gin.Context) {
+	codeID, err := strconv.ParseInt(c.Param("id"), 10, 64)
+	if err != nil {
+		response.BadRequest(c, "Invalid promo code ID")
+		return
+	}
+
+	err = h.promoService.Delete(c.Request.Context(), codeID)
+	if err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
+
+	response.Success(c, gin.H{"message": "Promo code deleted successfully"})
+}
+
+// GetUsages handles getting usage records for a promo code
+// GET /api/v1/admin/promo-codes/:id/usages
+func (h *PromoHandler) GetUsages(c *gin.Context) {
+	codeID, err := strconv.ParseInt(c.Param("id"), 10, 64)
+	if err != nil {
+		response.BadRequest(c, "Invalid promo code ID")
+		return
+	}
+
+	page, pageSize := response.ParsePagination(c)
+	params := pagination.PaginationParams{
+		Page:     page,
+		PageSize: pageSize,
+	}
+
+	usages, paginationResult, err := h.promoService.ListUsages(c.Request.Context(), codeID, params)
+	if err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
+
+	out := make([]dto.PromoCodeUsage, 0, len(usages))
+	for i := range usages {
+		out = append(out, *dto.PromoCodeUsageFromService(&usages[i]))
+	}
+	response.Paginated(c, out, paginationResult.Total, page, pageSize)
+}