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/store.go

@@ -158,6 +158,7 @@ func (s *Store) CreateSubscription(
 	userID, planID int64,
 	durationDays int,
 	latestSamePlan time.Time,
+	source string,
 ) (int64, error) {
 	now := time.Now().UTC()
 	base := now
@@ -168,8 +169,8 @@ func (s *Store) CreateSubscription(
 
 	res, err := tx.ExecContext(ctx,
 		`INSERT INTO subscriptions (user_id, plan_id, expires_at, source, created_at)
-		 VALUES (?, ?, ?, 'code', ?)`,
-		userID, planID, expiresAt, now)
+		 VALUES (?, ?, ?, ?, ?)`,
+		userID, planID, expiresAt, source, now)
 	if err != nil {
 		return 0, fmt.Errorf("store.CreateSubscription: %w", err)
 	}