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/modules/subscriptions/domain/repositories/subscription.repository.ts

@@ -0,0 +1,10 @@
+import { type SubscriptionEntity } from '../entities/subscription.entity';
+
+export const SUBSCRIPTION_REPOSITORY = Symbol('SUBSCRIPTION_REPOSITORY');
+
+export interface ISubscriptionRepository {
+  findById(id: string): Promise<SubscriptionEntity | null>;
+  findByUserId(userId: string): Promise<SubscriptionEntity | null>;
+  save(subscription: SubscriptionEntity): Promise<void>;
+  update(subscription: SubscriptionEntity): Promise<void>;
+}