fix(gate): triggerType nullable with no default per Section 2

- Producer must explicitly write its source (api/admin/scheduled/...)
- No database default to mask missing writes

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

prisma/migrations/20260620000008_fix_trigger_type_nullable/migration.sql

@@ -0,0 +1,5 @@
+-- M-AI-02-GATE Section 2: triggerType 统一为 nullable 无默认值
+-- Producer 必须显式写自己的来源，不得依赖数据库默认 'api' 掩盖缺失写入
+
+ALTER TABLE `AiAnalysisJob`
+  MODIFY COLUMN `triggerType` VARCHAR(32) NULL;