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/events/subscription-cancelled.event.ts

@@ -0,0 +1,13 @@
+import { type DomainEvent } from '@modules/shared/domain/domain-event';
+import { type PlanTier } from '@prisma/client';
+
+export class SubscriptionCancelledEvent implements DomainEvent {
+  readonly eventName = 'subscription.cancelled';
+  readonly occurredAt = new Date();
+
+  constructor(
+    public readonly aggregateId: string,
+    public readonly userId: string,
+    public readonly planTier: PlanTier,
+  ) {}
+}

apps/api/src/modules/subscriptions/domain/events/subscription-created.event.ts

@@ -0,0 +1,13 @@
+import { type DomainEvent } from '@modules/shared/domain/domain-event';
+import { type PlanTier } from '@prisma/client';
+
+export class SubscriptionCreatedEvent implements DomainEvent {
+  readonly eventName = 'subscription.created';
+  readonly occurredAt = new Date();
+
+  constructor(
+    public readonly aggregateId: string,
+    public readonly userId: string,
+    public readonly planTier: PlanTier,
+  ) {}
+}

apps/api/src/modules/subscriptions/domain/events/subscription-upgraded.event.ts

@@ -0,0 +1,14 @@
+import { type DomainEvent } from '@modules/shared/domain/domain-event';
+import { type PlanTier } from '@prisma/client';
+
+export class SubscriptionUpgradedEvent implements DomainEvent {
+  readonly eventName = 'subscription.upgraded';
+  readonly occurredAt = new Date();
+
+  constructor(
+    public readonly aggregateId: string,
+    public readonly userId: string,
+    public readonly fromTier: PlanTier,
+    public readonly toTier: PlanTier,
+  ) {}
+}