Migration and startup invariants

Identity

Drizzle migration artifacts, assertMigrationsReady, generated boot, and container command.

db:generate creates migration SQL plus Drizzle journal/snapshot state, and those artifacts are reviewed before deployment.
db:check validates the Drizzle snapshot chain. It does not inspect the live database or prove that a migration was applied.
One deployment job applies pending migrations before new application replicas serve traffic.
When registered tables exist, server startup refuses:
- a missing or unreadable migration directory;
- a migration file present on disk but absent from the applied ledger;
- an applied ledger entry whose migration is missing from disk; or
- an applied migration whose on-disk hash changed.
Server startup validates this applied-ledger readiness and never applies migrations from boot or request handling.
A failed migration or readiness mismatch is a startup/release failure.
Database backup and application rollback do not automatically reverse destructive SQL.