test(db): add migration 020 to bootstrap expected-migrations list

Migration 020 (arbiter_intervention_count) was added without updating the
hardcoded expected list in bootstrap.test.ts, breaking its schema-migration
assertions. Add the entry so the canonical `bun run test` suite passes.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/db/bootstrap.test.ts

@@ -44,6 +44,7 @@ function getExpectedSchemaMigrations(): Array<{
     { version: 17, name: 'scheduled_task_room_role' },
     { version: 18, name: 'paired_turn_output_attachments' },
     { version: 19, name: 'turn_progress_text_recovery' },
+    { version: 20, name: 'arbiter_intervention_count' },
   ];
 }