Add styled notification emails and round-attached notifications

- Add 15+ styled email templates matching existing invite email design
- Wire up notification triggers in all routers (assignment, round, project, mentor, application, onboarding)
- Add test email button for each notification type in admin settings
- Add round-attached notifications: admins can configure which notification to send when projects enter a round
- Fall back to status-based notifications when round has no configured notification

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

prisma/schema.prisma

@@ -385,6 +385,9 @@ model Round {
   requiredReviews Int         @default(3) // Min evaluations per project
   settingsJson    Json?       @db.JsonB   // Grace periods, visibility rules, etc.
 
+  // Notification sent to project team when they enter this round
+  entryNotificationType String?  // e.g., "ADVANCED_SEMIFINAL", "ADVANCED_FINAL"
+
   createdAt DateTime @default(now())
   updatedAt DateTime @updatedAt