feat: add subscription API endpoints and grace period job - M3 (refs #55)

API Endpoints (all authenticated):
- GET /api/subscriptions - current subscription status
- POST /api/subscriptions/checkout - create Stripe subscription
- POST /api/subscriptions/cancel - schedule cancellation at period end
- POST /api/subscriptions/reactivate - cancel pending cancellation
- PUT /api/subscriptions/payment-method - update payment method
- GET /api/subscriptions/invoices - billing history

Grace Period Job:
- Daily cron at 2:30 AM to check expired grace periods
- Downgrades to free tier when 30-day grace period expires
- Syncs tier to user_profiles.subscription_tier

Email Templates:
- payment_failed_immediate (first failure)
- payment_failed_7day (7 days before grace ends)
- payment_failed_1day (1 day before grace ends)

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/features/subscriptions/domain/subscriptions.service.ts

@@ -599,6 +599,25 @@ export class SubscriptionsService {
     return 'free';
   }
 
+  /**
+   * Get invoices for a user's subscription
+   */
+  async getInvoices(userId: string): Promise<any[]> {
+    try {
+      const subscription = await this.repository.findByUserId(userId);
+      if (!subscription?.stripeCustomerId) {
+        return [];
+      }
+      return this.stripeClient.listInvoices(subscription.stripeCustomerId);
+    } catch (error: any) {
+      logger.error('Failed to get invoices', {
+        userId,
+        error: error.message,
+      });
+      throw error;
+    }
+  }
+
   /**
    * Map subscription entity to response DTO
    */