feat(server): codes.GrantPaidSubscriptionTx 提炼订阅叠加语义供 pay 复用

CreateSubscription/applySubscription 加 source 参数(兑换路径传 "code"
零行为变化);新导出 GrantPaidSubscriptionTx(ctx, tx, ...) 供 pay 侧在
调用方事务内以 source='pay' 复用同一段叠加语义(同 plan 活跃订阅原地
延长,否则新建行),写 pay_grant 审计。

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013nMthbVEmQquxBRKb9Fj8u

server/internal/codes/paygrant.go

@@ -0,0 +1,31 @@
+package codes
+
+import (
+	"context"
+	"database/sql"
+	"encoding/json"
+	"fmt"
+	"time"
+)
+
+// GrantPaidSubscriptionTx 在调用方事务里执行支付驱动的订阅开通/续期。
+// 与兑换码走同一段叠加语义(applySubscription):同 plan 有活跃订阅则原地
+// 延长 max(expires,now)+days,否则新建 source='pay' 的一行。ref 为审计
+// 追踪串(如 "pay:<out_trade_no>"),审计写失败不影响事务(与兑换一致)。
+func (svc *Service) GrantPaidSubscriptionTx(
+	ctx context.Context, tx *sql.Tx, userID int64, plan PlanCode, days int, ref string,
+) (subID int64, expiresAt time.Time, err error) {
+	planID, err := svc.store.GetPlanIDTx(ctx, tx, plan)
+	if err != nil {
+		return 0, time.Time{}, err
+	}
+	subID, expiresAt, err = svc.applySubscription(ctx, tx, userID, planID, days, "pay")
+	if err != nil {
+		return 0, time.Time{}, err
+	}
+	meta, _ := json.Marshal(map[string]any{
+		"plan": string(plan), "duration_days": days, "sub_id": subID,
+	})
+	_ = svc.store.WriteAuditLog(ctx, tx, fmt.Sprintf("user:%d", userID), "pay_grant", ref, string(meta))
+	return subID, expiresAt, nil
+}