fix(billing): return default free-plan stub when no subscription exists

Previously GET /api/v1/billing/subscription threw 404 for tenants with no
subscription, causing React Query error state on the Plans and Overview pages.
Now returns a graceful default response so the UI renders without errors.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/services/billing-service/src/interfaces/rest/controllers/subscription.controller.ts

@@ -1,5 +1,5 @@
 import {
-  Controller, Get, Post, Body, Headers, HttpCode, HttpStatus, NotFoundException,
+  Controller, Get, Post, Body, Headers, HttpCode, HttpStatus,
 } from '@nestjs/common';
 import { SubscriptionRepository } from '../../../infrastructure/repositories/subscription.repository';
 import { PlanRepository } from '../../../infrastructure/repositories/plan.repository';
@@ -21,7 +21,21 @@ export class SubscriptionController {
   @Get('subscription')
   async getSubscription(@Headers('x-tenant-id') tenantId: string) {
     const sub = await this.subscriptionRepo.findByTenantId(tenantId);
-    if (!sub) throw new NotFoundException('No active subscription');
+    if (!sub) {
+      // No subscription yet — return a default free-plan stub so the UI renders gracefully
+      const freePlan = await this.planRepo.findByName('free');
+      return {
+        id: null,
+        status: 'none',
+        plan: freePlan ? { name: freePlan.name, displayName: freePlan.displayName } : { name: 'free', displayName: 'Free' },
+        nextPlan: null,
+        currentPeriodStart: null,
+        currentPeriodEnd: null,
+        trialEndsAt: null,
+        cancelAtPeriodEnd: false,
+        cancelledAt: null,
+      };
+    }
 
     const plan = await this.planRepo.findById(sub.planId);
     const nextPlan = sub.nextPlanId ? await this.planRepo.findById(sub.nextPlanId) : null;