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/api/subscriptions.routes.ts

@@ -0,0 +1,51 @@
+/**
+ * @ai-summary Fastify routes for subscriptions API
+ * @ai-context Route definitions with authentication for subscription management
+ */
+
+import { FastifyInstance, FastifyPluginOptions } from 'fastify';
+import { FastifyPluginAsync } from 'fastify';
+import { SubscriptionsController } from './subscriptions.controller';
+
+export const subscriptionsRoutes: FastifyPluginAsync = async (
+  fastify: FastifyInstance,
+  _opts: FastifyPluginOptions
+) => {
+  const subscriptionsController = new SubscriptionsController();
+
+  // GET /api/subscriptions - Get current subscription
+  fastify.get('/subscriptions', {
+    preHandler: [fastify.authenticate],
+    handler: subscriptionsController.getSubscription.bind(subscriptionsController)
+  });
+
+  // POST /api/subscriptions/checkout - Create Stripe checkout session
+  fastify.post('/subscriptions/checkout', {
+    preHandler: [fastify.authenticate],
+    handler: subscriptionsController.createCheckout.bind(subscriptionsController)
+  });
+
+  // POST /api/subscriptions/cancel - Schedule cancellation
+  fastify.post('/subscriptions/cancel', {
+    preHandler: [fastify.authenticate],
+    handler: subscriptionsController.cancelSubscription.bind(subscriptionsController)
+  });
+
+  // POST /api/subscriptions/reactivate - Cancel pending cancellation
+  fastify.post('/subscriptions/reactivate', {
+    preHandler: [fastify.authenticate],
+    handler: subscriptionsController.reactivateSubscription.bind(subscriptionsController)
+  });
+
+  // PUT /api/subscriptions/payment-method - Update payment method
+  fastify.put('/subscriptions/payment-method', {
+    preHandler: [fastify.authenticate],
+    handler: subscriptionsController.updatePaymentMethod.bind(subscriptionsController)
+  });
+
+  // GET /api/subscriptions/invoices - Get billing history
+  fastify.get('/subscriptions/invoices', {
+    preHandler: [fastify.authenticate],
+    handler: subscriptionsController.getInvoices.bind(subscriptionsController)
+  });
+};