feat(migrate): migration_map idempotency ledger

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/db/repos/migration_map.js

@@ -0,0 +1,15 @@
+import { pool } from '../pool.js';
+
+export async function seen(source, source_id, entity_type) {
+  const { rows: [r] } = await pool.query(
+    `SELECT entity_id FROM migration_map WHERE source=$1 AND source_id=$2 AND entity_type=$3`,
+    [source, source_id, entity_type]);
+  return r ? r.entity_id : null;
+}
+
+export async function record(source, source_id, entity_type, entity_id) {
+  await pool.query(
+    `INSERT INTO migration_map(source, source_id, entity_type, entity_id) VALUES($1,$2,$3,$4)
+     ON CONFLICT (source, source_id, entity_type) DO NOTHING`,
+    [source, source_id, entity_type, entity_id]);
+}