feat(api): use profiles.displayName instead of email in admin review/comment joins

[cdcore09]

Summary

Admin detail pages (events + announcements) currently join users.email to surface reviewer / comment-author identity. Display name is the right field for human-readable attribution; fall back to email only when display name is null.

Requirements

• Resolve reviewer/author display name from profiles.displayName (joined on profiles.userId = users.id) in:
  • packages/api/src/routes/admin/events/byId.ts
  • packages/api/src/routes/admin/announcements/byId.ts
• Fall back to email when displayName is null
• Update admin detail pages to render reviewerName / authorName directly (no UI changes expected since fields are already optional)

Context

Flagged in the Plan 3 review. Audit panel and review history will also benefit (same pattern).