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/db/admin.go

@@ -0,0 +1,135 @@
+package db
+
+import (
+	"context"
+	"encoding/json"
+)
+
+func (r *PGRepository) ListAllTenants(ctx context.Context, limit, offset int) ([]TenantRecord, int, error) {
+	var total int
+	err := r.pool.QueryRow(ctx, `SELECT COUNT(*) FROM tenants`).Scan(&total)
+	if err != nil {
+		return nil, 0, err
+	}
+
+	rows, err := r.pool.Query(ctx, `
+		SELECT id, slug, name, preset, locale, timezone, plan_code, subscription_status,
+		       COALESCE(billing_provider, 'stripe'), billing_customer_id, billing_subscription_id
+		FROM tenants
+		ORDER BY created_at DESC
+		LIMIT $1 OFFSET $2
+	`, limit, offset)
+	if err != nil {
+		return nil, 0, err
+	}
+	defer rows.Close()
+
+	var tenants []TenantRecord
+	for rows.Next() {
+		var t TenantRecord
+		if err := rows.Scan(
+			&t.ID, &t.Slug, &t.Name, &t.Preset, &t.Locale, &t.Timezone,
+			&t.PlanCode, &t.SubscriptionStatus, &t.BillingProvider,
+			&t.BillingCustomerID, &t.BillingSubscription,
+		); err != nil {
+			return nil, 0, err
+		}
+		tenants = append(tenants, t)
+	}
+
+	return tenants, total, nil
+}
+
+func (r *PGRepository) ListAllUsers(ctx context.Context, limit, offset int) ([]UserRecord, int, error) {
+	var total int
+	err := r.pool.QueryRow(ctx, `SELECT COUNT(*) FROM users`).Scan(&total)
+	if err != nil {
+		return nil, 0, err
+	}
+
+	rows, err := r.pool.Query(ctx, `
+		SELECT id, email, name, email_verified, provider, role, created_at, last_login_at
+		FROM users
+		ORDER BY created_at DESC
+		LIMIT $1 OFFSET $2
+	`, limit, offset)
+	if err != nil {
+		return nil, 0, err
+	}
+	defer rows.Close()
+
+	var users []UserRecord
+	for rows.Next() {
+		var u UserRecord
+		if err := rows.Scan(
+			&u.ID, &u.Email, &u.Name, &u.EmailVerified, &u.Provider, &u.Role,
+			&u.CreatedAt, &u.LastLoginAt,
+		); err != nil {
+			return nil, 0, err
+		}
+		users = append(users, u)
+	}
+
+	return users, total, nil
+}
+
+func (r *PGRepository) GetPlatformStats(ctx context.Context) (PlatformStats, error) {
+	var stats PlatformStats
+
+	// Total tenants
+	r.pool.QueryRow(ctx, `SELECT COUNT(*) FROM tenants`).Scan(&stats.TotalTenants)
+
+	// Total users
+	r.pool.QueryRow(ctx, `SELECT COUNT(*) FROM users`).Scan(&stats.TotalUsers)
+
+	// Active subscriptions
+	r.pool.QueryRow(ctx, `
+		SELECT COUNT(*) FROM billing_snapshots 
+		WHERE status IN ('active', 'trialing')
+	`).Scan(&stats.ActiveSubscriptions)
+
+	// Trial subscriptions
+	r.pool.QueryRow(ctx, `
+		SELECT COUNT(*) FROM billing_snapshots 
+		WHERE status = 'trialing'
+	`).Scan(&stats.TrialSubscriptions)
+
+	// Bookings this month
+	r.pool.QueryRow(ctx, `
+		SELECT COUNT(*) FROM bookings 
+		WHERE created_at >= date_trunc('month', CURRENT_DATE)
+	`).Scan(&stats.BookingsThisMonth)
+
+	return stats, nil
+}
+
+func (r *PGRepository) CreateAdminAuditLog(ctx context.Context, params AdminAuditLogParams) error {
+	var detailsJSON []byte
+	var err error
+	if params.Details != nil {
+		detailsJSON, err = json.Marshal(params.Details)
+		if err != nil {
+			return err
+		}
+	}
+
+	_, err = r.pool.Exec(ctx, `
+		INSERT INTO admin_audit_log (admin_user_id, action, resource_type, resource_id, details, ip_address, user_agent)
+		VALUES ($1, $2, $3, $4, $5, $6, $7)
+	`, nullableUUID(params.AdminUserID), params.Action, params.ResourceType, params.ResourceID, detailsJSON, params.IPAddress, params.UserAgent)
+	return err
+}
+
+func (r *PGRepository) UpdateUserRole(ctx context.Context, userID, role string) error {
+	_, err := r.pool.Exec(ctx, `
+		UPDATE users SET role = $1, updated_at = NOW() WHERE id = $2
+	`, role, userID)
+	return err
+}
+
+func nullableUUID(s string) interface{} {
+	if s == "" {
+		return nil
+	}
+	return s
+}