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.

.gitignore

@@ -13,5 +13,9 @@ coverage/
 # Logs
 *.log
 
+# Transient host-side probe file written by tests/database-postgres-ledger.test.mjs
+# into the database-postgres package (removed in its finally block)
+.ledger-probe-*.mjs
+
 # OS / editor
 .DS_Store