feat(migrate): migration_map idempotency ledger

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

lib/db/migrations/018_migration_map.sql

@@ -0,0 +1,11 @@
+-- 018_migration_map.sql — idempotency ledger for void-migrate. Maps a source row
+-- to the Void 2 entity it created, so re-running an import never duplicates.
+CREATE TABLE migration_map (
+  id          uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  source      text NOT NULL,                 -- 'void1' | 'bookstack' | 'karakeep' | 'plans'
+  source_id   text NOT NULL,                 -- e.g. 'wiki_pages:42'
+  entity_type text NOT NULL,                 -- 'page' | 'project' | 'task' | 'conversation' | 'message'
+  entity_id   uuid NOT NULL,
+  created_at  timestamptz NOT NULL DEFAULT now(),
+  UNIQUE (source, source_id, entity_type)
+);

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]);
+}