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)
+);