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);

prisma/schema.prisma

@@ -302,6 +302,9 @@ model User {
   institution    String? // User's institution/organization
   metadataJson   Json?      @db.JsonB
 
+  // Mentor onboarding email idempotency: stamped once when MENTOR role is first added.
+  mentorOnboardingSentAt DateTime?
+
   // Profile
   bio                  String? // User bio for matching with project descriptions
   profileImageKey      String? // Storage key (e.g., "avatars/user123/1234567890.jpg")