fix(backend): 架构质量改进批次二 (#32-37)

- #32 License 激活迁移到 license_devices 表：Activate/Verify/Deactivate 全部改用
  license_devices，新增 max_devices 校验和 GET /license/devices 端点；
  Activate 现在校验 shop_id 防跨租户激活
- #33 checkInventory 从 StockOutHandler 移到 StockService.CheckInventoryAvailability
- #34 新增 util/response.go 统一错误响应工具（RespondError/RespondSuccess/RespondCreated）
- #35 生产模式 CORS Origin='*' 启动时 Fatal
- #36 生产模式 License 私钥未配置启动时 Fatal
- #37 新增 util/page.go ValidatePageSize，应用到 partner/product/stock_in/stock_out handler

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/util/page.go

@@ -0,0 +1,9 @@
+package util
+
+// ValidatePageSize clamps pageSize into [1, maxSize], returning defaultSize when out of range.
+func ValidatePageSize(pageSize, defaultSize, maxSize int) int {
+	if pageSize < 1 || pageSize > maxSize {
+		return defaultSize
+	}
+	return pageSize
+}

backend/internal/util/response.go

@@ -0,0 +1,22 @@
+package util
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+)
+
+// RespondError writes a structured error response: {"code": code, "message": msg}.
+func RespondError(c *gin.Context, status int, code, msg string) {
+	c.JSON(status, gin.H{"code": code, "message": msg})
+}
+
+// RespondSuccess writes {"data": data} with HTTP 200.
+func RespondSuccess(c *gin.Context, data interface{}) {
+	c.JSON(http.StatusOK, gin.H{"data": data})
+}
+
+// RespondCreated writes {"data": data} with HTTP 201.
+func RespondCreated(c *gin.Context, data interface{}) {
+	c.JSON(http.StatusCreated, gin.H{"data": data})
+}