Add onboarding email campaign system with post-verification welcome email

Implements automated onboarding emails to encourage user engagement:
- Post-verification welcome email with 5 tips (sent after email verification)
- "No Residence" email (2+ days after registration with no property)
- "No Tasks" email (5+ days after first residence with no tasks)

Key features:
- Each onboarding email type sent only once per user (enforced by unique constraint)
- Email open tracking via tracking pixel endpoint
- Daily scheduled job at 10:00 AM UTC to process eligible users
- Admin panel UI for viewing sent emails, stats, and manual sending
- Admin can send any email type to users from the user detail Testing section

New files:
- internal/models/onboarding_email.go - Database model with tracking
- internal/services/onboarding_email_service.go - Business logic and eligibility queries
- internal/handlers/tracking_handler.go - Email open tracking endpoint
- internal/admin/handlers/onboarding_handler.go - Admin API endpoints
- admin/src/app/(dashboard)/onboarding-emails/ - Admin UI pages

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

internal/database/database.go

@@ -152,12 +152,18 @@ func Migrate() error {
 		&models.FeatureBenefit{},
 		&models.Promotion{},
 		&models.TierLimits{},
+
+		// Onboarding email tracking
+		&models.OnboardingEmail{},
 	)
 
 	if err != nil {
 		return fmt.Errorf("failed to run migrations: %w", err)
 	}
 
+	// Add unique constraint for onboarding emails (one email type per user)
+	db.Exec(`CREATE UNIQUE INDEX IF NOT EXISTS idx_onboarding_emails_user_type ON onboarding_emails(user_id, email_type)`)
+
 	// Run GoAdmin migrations
 	if err := migrateGoAdmin(); err != nil {
 		return fmt.Errorf("failed to run GoAdmin migrations: %w", err)