feat(M-AI-02-08): AiUsageLog job relation + credential fields

- 5 new columns (jobId, attemptNo, credentialMode, errorCode, providerRequestId)
- FK ON DELETE SET NULL (preserve audit trail)
- 3 new indexes: (jobId,attemptNo), (provider,model,createdAt), (errorCode,createdAt)

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

prisma/migrations/20260620000005_m_ai_02_08_ai_usage_log_expand/migration.sql

@@ -0,0 +1,21 @@
+-- M-AI-02-08: 扩展 AiUsageLog 的 Job、尝试次数与凭据归属
+
+-- 1. 新增 Job 关联与凭据归属字段
+ALTER TABLE `AiUsageLog`
+  ADD COLUMN `jobId` VARCHAR(191) NULL AFTER `errorMessage`,
+  ADD COLUMN `attemptNo` INT NOT NULL DEFAULT 0 AFTER `jobId`,
+  ADD COLUMN `credentialMode` VARCHAR(32) NOT NULL DEFAULT 'platform_key' AFTER `attemptNo`,
+  ADD COLUMN `errorCode` VARCHAR(100) NULL AFTER `credentialMode`,
+  ADD COLUMN `providerRequestId` VARCHAR(255) NULL AFTER `errorCode`;
+
+-- 2. FK: Job 删除时 SetNull UsageLog（保留审计记录）
+ALTER TABLE `AiUsageLog`
+  ADD CONSTRAINT `AiUsageLog_jobId_fkey`
+    FOREIGN KEY (`jobId`) REFERENCES `AiAnalysisJob`(`id`)
+    ON DELETE SET NULL ON UPDATE CASCADE;
+
+-- 3. 索引
+ALTER TABLE `AiUsageLog`
+  ADD INDEX `AiUsageLog_jobId_attemptNo_idx` (`jobId`, `attemptNo`),
+  ADD INDEX `AiUsageLog_provider_model_createdAt_idx` (`provider`, `model`, `createdAt`),
+  ADD INDEX `AiUsageLog_errorCode_createdAt_idx` (`errorCode`, `createdAt`);