feat(backend): 授权改为时长兑换券体系 + 退役 ed25519/HMAC + 平台生码工具

- 新增 license_codes 码池表 + model.LicenseCode；licenses 加 tier 档位列
- LicenseService.Redeem：单事务 FOR UPDATE 校验码未用 → 时长叠加(可叠加，0=永久)
  → 写 type/tier/max_devices → 绑设备(超限整笔回滚) → 标记已用 → 即时失效 phase 缓存
  路由仍 POST /license/activate，客户端零破坏
- util.GenerateRedeemCode/NormalizeCode：JIUKU-XXXX-XXXX 短码(crypto/rand)
- cmd/gencode：平台批量生成兑换码并落库；删除 cmd/issue、cmd/genkey
- 退役 ed25519 + HMAC：删 util/license_key、GenerateKey、License 全部 config 字段
  及生产启动私钥校验；trial 改直接建行(无需私钥、去 Fatal)
- tier 档位钩子默认 standard，分档消费模式后续设计
- 测试：Redeem 全场景(叠加/过期重置/永久/一码一次/无效/设备上限回滚)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/cmd/gencode/main.go

@@ -0,0 +1,90 @@
+// gencode — 平台方批量生成兑换券（激活码）并写入码池 license_codes。
+//
+// 用法（在 backend/ 目录下执行，读 config/env 取数据库 DSN）：
+//
+//	go run ./cmd/gencode -type annual -days 365 -count 100 -batch 2026-summer
+//	go run ./cmd/gencode -days 30 -count 10 -note "试用补偿"
+//	go run ./cmd/gencode -type lifetime -days 0 -count 1        # 永久授权码
+//
+// 生成的码以 JIUKU-XXXX-XXXX 格式打印（status=unused），分发给用户在 App 内兑换。
+package main
+
+import (
+	"flag"
+	"fmt"
+	stdlog "log"
+	"os"
+	"time"
+
+	"gorm.io/driver/mysql"
+	"gorm.io/gorm"
+	"gorm.io/gorm/logger"
+
+	"github.com/wangjia/jiu/backend/config"
+	"github.com/wangjia/jiu/backend/internal/model"
+	"github.com/wangjia/jiu/backend/internal/util"
+)
+
+func main() {
+	licType := flag.String("type", "annual", "计费/时长标签：trial|monthly|annual|lifetime")
+	tier := flag.String("tier", "standard", "档位（当前仅 standard）")
+	days := flag.Int("days", 365, "授予时长（天）；0 = 永久")
+	devices := flag.Int("devices", 0, "授予设备上限；0 = 兑换时不改变门店现值")
+	count := flag.Int("count", 1, "生成数量")
+	batch := flag.String("batch", "", "发放批次/活动名（便于追踪）")
+	note := flag.String("note", "", "备注")
+	flag.Parse()
+
+	if *count < 1 {
+		fmt.Fprintln(os.Stderr, "error: -count 必须 >= 1")
+		os.Exit(1)
+	}
+
+	config.Load()
+	db, err := gorm.Open(mysql.Open(config.C.Database.DSN), &gorm.Config{
+		Logger: logger.New(
+			stdlog.New(os.Stdout, "\r\n", stdlog.LstdFlags),
+			logger.Config{LogLevel: logger.Warn, IgnoreRecordNotFoundError: true},
+		),
+	})
+	if err != nil {
+		stdlog.Fatalf("连接数据库失败: %v", err)
+	}
+	if err := db.AutoMigrate(&model.LicenseCode{}); err != nil {
+		stdlog.Fatalf("迁移 license_codes 失败: %v", err)
+	}
+
+	display := make([]string, 0, *count)
+	for i := 0; i < *count; i++ {
+		// 重试避免极小概率的唯一冲突
+		var shown string
+		for attempt := 0; attempt < 5; attempt++ {
+			shown = util.GenerateRedeemCode()
+			lc := model.LicenseCode{
+				Code:         util.NormalizeCode(shown),
+				Type:         *licType,
+				Tier:         *tier,
+				DurationDays: *days,
+				MaxDevices:   *devices,
+				Status:       "unused",
+				Batch:        *batch,
+				Note:         *note,
+			}
+			err := db.Create(&lc).Error
+			if err == nil {
+				break
+			}
+			if attempt == 4 {
+				stdlog.Fatalf("写入兑换码失败（连续冲突）: %v", err)
+			}
+		}
+		display = append(display, shown)
+	}
+
+	fmt.Printf("\n✅ 已生成 %d 个兑换码（type=%s tier=%s 时长=%d天 设备=%d batch=%q）于 %s：\n\n",
+		*count, *licType, *tier, *days, *devices, *batch, time.Now().Format("2006-01-02 15:04:05"))
+	for _, c := range display {
+		fmt.Println("   " + c)
+	}
+	fmt.Println()
+}