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/core/scheduler/index.ts

@@ -19,6 +19,10 @@ import {
   processAuditLogCleanup,
   setAuditLogCleanupJobPool,
 } from '../../features/audit-log/jobs/cleanup.job';
+import {
+  processGracePeriodExpirations,
+  setGracePeriodJobPool,
+} from '../../features/subscriptions/jobs/grace-period.job';
 import { pool } from '../config/database';
 
 let schedulerInitialized = false;
@@ -38,6 +42,9 @@ export function initializeScheduler(): void {
   // Initialize audit log cleanup job pool
   setAuditLogCleanupJobPool(pool);
 
+  // Initialize grace period job pool
+  setGracePeriodJobPool(pool);
+
   // Daily notification processing at 8 AM
   cron.schedule('0 8 * * *', async () => {
     logger.info('Running scheduled notification job');
@@ -67,6 +74,23 @@ export function initializeScheduler(): void {
     }
   });
 
+  // Grace period expiration check at 2:30 AM daily
+  cron.schedule('30 2 * * *', async () => {
+    logger.info('Running grace period expiration job');
+    try {
+      const result = await processGracePeriodExpirations();
+      logger.info('Grace period job completed', {
+        processed: result.processed,
+        downgraded: result.downgraded,
+        errors: result.errors.length,
+      });
+    } catch (error) {
+      logger.error('Grace period job failed', {
+        error: error instanceof Error ? error.message : String(error)
+      });
+    }
+  });
+
   // Check for scheduled backups every minute
   cron.schedule('* * * * *', async () => {
     logger.debug('Checking for scheduled backups');
@@ -120,7 +144,7 @@ export function initializeScheduler(): void {
   });
 
   schedulerInitialized = true;
-  logger.info('Cron scheduler initialized - notification (8 AM), account purge (2 AM), audit log cleanup (3 AM), backup check (every min), retention cleanup (4 AM)');
+  logger.info('Cron scheduler initialized - notification (8 AM), account purge (2 AM), grace period (2:30 AM), audit log cleanup (3 AM), backup check (every min), retention cleanup (4 AM)');
 }
 
 export function isSchedulerInitialized(): boolean {