feat(db): add User.mentorOnboardingSentAt for one-shot onboarding email (§D)

Single nullable DateTime column. No backfill. Catalog-only ALTER TABLE —
sub-millisecond on PostgreSQL regardless of table size. The column is
unused until the bulk role-update flow wires it up as an idempotency
stamp for the mentor-onboarding email.

Plan: docs/superpowers/plans/2026-04-28-pr6-multi-role-and-workspace-previews.md
Spec: docs/superpowers/specs/2026-04-28-mentor-round-readiness-design.md §D

prisma/migrations/20260428135613_add_mentor_onboarding_sent_at/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "User" ADD COLUMN     "mentorOnboardingSentAt" TIMESTAMP(3);