Turn the three-service surprise into a rollout map

Before writing much code, spend twenty minutes drawing the change as four lanes: the three services and the database. Mark which versions can coexist, which component owns the backfill, and the exact observation that makes each old path safe to remove. The useful design question is not merely “what must change?” but “what is the smallest independently deployable expand, migrate, and contract sequence?” This should expose whether the data migration is genuinely required up front or can run after compatible readers and writers are live.