[plan] Merge Sign-ups into Customers Roster

_Automatically composed from the parent epic at dispatch time._

Split along the Architect's C1–C4 boundaries into 4 coders across 2 phases. Phase 1: roster (10410) and terms gate (10411). Phase 2: operator actions (10412) and purge retirement (10413). The latter has a migration-wave edge. Fixed the critic's findings: resend audit added, draft dev oracles made best-effort plus a re-run in 10412. Memory: none applied.