Migration state was scattered across PRAGMA-table_info guards, sentinel marker
tables (_genius_search_fix_applied, ...) and metadata-flag rows
(id_columns_migrated, ...), with no single source of truth and no schema
version — so a half-migrated DB was undetectable.
Add a non-gating backstop: a schema_migrations(name, applied_at) ledger plus a
_sync_migration_ledger pass (runs last in init) that back-fills the ledger from
the existing signals and stamps PRAGMA user_version. ADDITIVE only — existing
migrations keep their own idempotency gates; nothing decides whether a
migration runs based on the ledger or the version. New one-time migrations call
_record_migration (the genres migration already does).
Tests: tests/test_db_migration_ledger.py — table exists, user_version stamped,
record idempotent, genres recorded on fresh init, backfill from flag + marker,
absent signals not recorded.
