🐛 fix(gifts): explicitly set nullable fields to null in DB inserts

Drizzle ORM requires explicit null values for optional fields instead of
leaving them undefined. Added explicit null for organizationId,
idempotencyKey, portionNumber, and creditTransactionId.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

services/mana-core-auth/src/gifts/services/gift-code.service.ts

@@ -359,6 +359,8 @@ export class GiftCodeService {
 						redeemerUserId: userId,
 						status: 'failed_wrong_user',
 						creditsReceived: 0,
+						portionNumber: null,
+						creditTransactionId: null,
 						sourceAppId: dto.sourceAppId ?? null,
 					});
 
@@ -383,6 +385,8 @@ export class GiftCodeService {
 						redeemerUserId: userId,
 						status: 'failed_wrong_answer',
 						creditsReceived: 0,
+						portionNumber: null,
+						creditTransactionId: null,
 						sourceAppId: dto.sourceAppId ?? null,
 					});
 
@@ -463,7 +467,9 @@ export class GiftCodeService {
 					balanceAfter: newBalance,
 					appId: dto.sourceAppId || 'gift',
 					description: `Gift received: ${giftCode.code}`,
+					organizationId: null,
 					metadata: { giftCodeId: giftCode.id, portionNumber },
+					idempotencyKey: null,
 					completedAt: new Date(),
 				})
 				.returning();