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/external/stripe/stripe.client.ts

@@ -323,4 +323,32 @@ export class StripeClient {
       throw error;
     }
   }
+
+  /**
+   * List invoices for a customer
+   */
+  async listInvoices(customerId: string): Promise<any[]> {
+    try {
+      logger.info('Listing Stripe invoices', { customerId });
+
+      const invoices = await this.stripe.invoices.list({
+        customer: customerId,
+        limit: 20,
+      });
+
+      logger.info('Stripe invoices retrieved', {
+        customerId,
+        count: invoices.data.length
+      });
+
+      return invoices.data;
+    } catch (error: any) {
+      logger.error('Failed to list Stripe invoices', {
+        customerId,
+        error: error.message,
+        code: error.code,
+      });
+      throw error;
+    }
+  }
 }