feat(core): consolidate auth service into backend and implement stripe billing

This commit performs a major architectural refactor by migrating the standalone `auth-service` into the main `backend` application, enabling a unified codebase and simplified deployment. It also introduces comprehensive Stripe billing support and a new administrative dashboard.

Key changes:
- **Architecture**: Deleted `apps/auth-service` and integrated its functionality (JWT, magic links, OAuth, user management) into `apps/backend`.
- **Billing**: Added Stripe integration to `backend`, supporting both monthly and yearly subscription cycles with automatic plan entitlement enforcement (e.g., location limits).
- **Admin Dashboard**: Implemented a new administrative service and API endpoints to manage tenants, users, and view platform-wide statistics.
- **Frontend**:
    - Added a new pricing page with monthly/yearly toggle and comparison table.
    - Integrated Stripe and Sentry for payments and error tracking.
    - Improved dashboard UX/UI and added i18n support for new features.
    - Enhanced the public booking flow with better validation and contact form integration.
- **Database**: Added migrations for users, magic links, password resets, OAuth states, admin audit logs, and refresh tokens.
- **DevOps**: Updated environment configurations for Railway and Vercel, and streamlined the project's `package.json` scripts.

apps/backend/internal/notifications/service.go

@@ -285,3 +285,77 @@ func (p smtpEmailProvider) Send(_ context.Context, message EmailMessage) (Delive
 		ExternalID: fmt.Sprintf("smtp-%d", time.Now().UnixNano()),
 	}, nil
 }
+
+// SendContactEmail sends a contact form submission to the business email
+func (s *Service) SendContactEmail(ctx context.Context, name, email, message string) error {
+	subject := fmt.Sprintf("Bookra Contact: Message from %s", name)
+	text := fmt.Sprintf("Name: %s\nEmail: %s\n\nMessage:\n%s", name, email, message)
+	html := fmt.Sprintf(
+		"<h2>New contact form submission</h2><p><strong>Name:</strong> %s</p><p><strong>Email:</strong> %s</p><p><strong>Message:</strong></p><p>%s</p>",
+		name, email, message,
+	)
+	msg := EmailMessage{
+		From:    s.cfg.EmailFrom,
+		To:      s.cfg.EmailFrom,
+		Subject: subject,
+		Text:    text,
+		HTML:    html,
+	}
+	_, err := s.emailProvider.Send(ctx, msg)
+	return err
+}
+
+func (s *Service) SendUsageWarning(ctx context.Context, tenantID string, locationCount, locationLimit, usagePercent int) error {
+	tenant, err := s.repo.GetTenantByID(ctx, tenantID)
+	if err != nil {
+		return fmt.Errorf("failed to get tenant: %w", err)
+	}
+
+	// Use a placeholder admin email - in production, would get from tenant owner
+	adminEmail := "admin@" + tenant.Slug + ".bookra.eu"
+
+	emailData := UsageNotificationData{
+		Type:          EmailTypeUsageWarning,
+		TenantName:    tenant.Name,
+		TenantSlug:    tenant.Slug,
+		BusinessEmail: s.cfg.EmailFrom,
+		AdminEmail:    adminEmail,
+		Locale:        tenant.Locale,
+		PlanCode:      tenant.PlanCode,
+		LocationCount: locationCount,
+		LocationLimit: locationLimit,
+		UsagePercent:  usagePercent,
+		UpgradeURL:    "https://bookra.eu/pricing",
+		DashboardURL:  "https://bookra.eu/dashboard",
+	}
+
+	msg := RenderUsageNotificationEmail(emailData)
+	_, err = s.emailProvider.Send(ctx, msg)
+	return err
+}
+
+func (s *Service) SendTrialEndingEmail(ctx context.Context, tenantID string, daysRemaining int) error {
+	tenant, err := s.repo.GetTenantByID(ctx, tenantID)
+	if err != nil {
+		return fmt.Errorf("failed to get tenant: %w", err)
+	}
+
+	// Use a placeholder admin email - in production, would get from tenant owner
+	adminEmail := "admin@" + tenant.Slug + ".bookra.eu"
+
+	emailData := UsageNotificationData{
+		Type:          EmailTypeTrialEnding,
+		TenantName:    tenant.Name,
+		TenantSlug:    tenant.Slug,
+		BusinessEmail: s.cfg.EmailFrom,
+		AdminEmail:    adminEmail,
+		Locale:        tenant.Locale,
+		PlanCode:      tenant.PlanCode,
+		UpgradeURL:    "https://bookra.eu/pricing",
+		DashboardURL:  "https://bookra.eu/dashboard",
+	}
+
+	msg := RenderUsageNotificationEmail(emailData)
+	_, err = s.emailProvider.Send(ctx, msg)
+	return err
+}