feat(subscriptions): add Subscriptions module with plans, quotas, and billing

- Add Subscription, Plan, UsageRecord domain entities
- Implement Create, Upgrade, Cancel subscription commands
- Add MeterUsage command for quota tracking
- Support 4 plan tiers: Free, Agent Pro, Investor, Enterprise
- Register SubscriptionsModule in AppModule

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/api/src/app.module.ts

@@ -4,6 +4,7 @@ import { ListingsModule } from '@modules/listings';
 import { SearchModule } from '@modules/search';
 import { NotificationsModule } from '@modules/notifications';
 import { PaymentsModule } from '@modules/payments';
+import { SubscriptionsModule } from '@modules/subscriptions';
 import { Module } from '@nestjs/common';
 import { APP_GUARD } from '@nestjs/core';
 import { CqrsModule } from '@nestjs/cqrs';
@@ -20,6 +21,7 @@ import { AppController } from './app.controller';
     SearchModule,
     NotificationsModule,
     PaymentsModule,
+    SubscriptionsModule,
 
     // ── Rate Limiting ──
     // Default: 60 requests per 60 seconds per IP