feat: add migration ledger to database-postgres (E00-S03-T03)

MigrationLedger over the package-owned pg Pool: ensure() creates the
schema_migrations table (version text PRIMARY KEY, applied_at timestamptz
NOT NULL DEFAULT now()) with idempotent DDL; record() inserts an applied
migration with a parameterized, idempotent statement (ON CONFLICT DO
NOTHING — a rerun never double-applies); has()/applied() read the ledger
back in apply order. Re-exported from the driver boundary (src/index.ts)
so no other package needs the pg driver to touch migration state.

packages/database-postgres/src/index.ts

@@ -8,13 +8,15 @@
  * package will expose — so no other package ever imports the driver directly.
  *
  * This module is the driver boundary: it imports the PostgreSQL driver (`pg`)
- * and Kysely and re-exports the pieces later stories build the adapter on
- * (migration ledger E00-S03-T03, advisory lock E00-S03-T04). Until then the
- * re-exports keep the driver reachable only from here — the isolation is real,
- * not a placeholder.
+ * and Kysely and re-exports the pieces the adapter is built on — the driver
+ * surface (E00-S03-T02) and the migration ledger (E00-S03-T03). The advisory
+ * lock (E00-S03-T04) and failure diagnostic (E00-S03-T05) land in later
+ * stories; until then the re-exports keep the driver reachable only from here
+ * — the isolation is real, not a placeholder.
  */
 
 import { Pool } from 'pg';
 import { Kysely, PostgresDialect } from 'kysely';
 
 export { Pool, Kysely, PostgresDialect };
+export { MigrationLedger, MIGRATION_LEDGER_TABLE } from './ledger.js';