Migration and startup invariants

Identity

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

Migration SQL is generated and reviewed before deployment.
One deployment job applies pending migrations before new application replicas serve traffic.
Server startup validates readiness and never migrates from request handling.
A failed migration or readiness mismatch is a startup/release failure.
Database backup and application rollback do not automatically reverse destructive SQL.