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/handlers/auth_handler.go

@@ -222,6 +222,15 @@ func (h *AuthHandler) VerifyEmail(c *gin.Context) {
 		return
 	}
 
+	// Send post-verification welcome email with tips (async)
+	if h.emailService != nil {
+		go func() {
+			if err := h.emailService.SendPostVerificationEmail(user.Email, user.FirstName); err != nil {
+				log.Error().Err(err).Str("email", user.Email).Msg("Failed to send post-verification email")
+			}
+		}()
+	}
+
 	c.JSON(http.StatusOK, responses.VerifyEmailResponse{
 		Message:  i18n.LocalizedMessage(c, "message.email_verified"),
 		Verified: true,