feat(#90): implement EVENT subscriptions for federation

Implement event pub/sub messaging for federation to enable real-time
event streaming between federated instances.

Features:
- Event subscription management (subscribe/unsubscribe)
- Event publishing to subscribed instances
- Event acknowledgment protocol
- Server-side event filtering based on subscriptions
- Full signature verification and connection validation

Implementation:
- FederationEventSubscription model for storing subscriptions
- EventService with complete event lifecycle management
- EventController with authenticated and public endpoints
- EventMessage, EventAck, and SubscriptionDetails types
- Comprehensive DTOs for all event operations

API Endpoints:
- POST /api/v1/federation/events/subscribe
- POST /api/v1/federation/events/unsubscribe
- POST /api/v1/federation/events/publish
- GET /api/v1/federation/events/subscriptions
- GET /api/v1/federation/events/messages
- POST /api/v1/federation/incoming/event (public)
- POST /api/v1/federation/incoming/event/ack (public)

Testing:
- 18 unit tests for EventService (89.09% coverage)
- 11 unit tests for EventController (83.87% coverage)
- All 29 tests passing
- Follows TDD red-green-refactor cycle

Technical Notes:
- Reuses existing FederationMessage model with eventType field
- Follows patterns from QueryService and CommandService
- Uses existing signature and connection infrastructure
- Supports hierarchical event type naming (e.g., "task.created")

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/api/src/federation/dto/event.dto.ts

@@ -0,0 +1,109 @@
+/**
+ * Event DTOs
+ *
+ * Data Transfer Objects for event subscription and publishing.
+ */
+
+import { IsString, IsNotEmpty, IsOptional, IsObject } from "class-validator";
+
+/**
+ * DTO for subscribing to an event type
+ */
+export class SubscribeToEventDto {
+  @IsString()
+  @IsNotEmpty()
+  connectionId!: string;
+
+  @IsString()
+  @IsNotEmpty()
+  eventType!: string;
+
+  @IsOptional()
+  @IsObject()
+  metadata?: Record<string, unknown>;
+}
+
+/**
+ * DTO for unsubscribing from an event type
+ */
+export class UnsubscribeFromEventDto {
+  @IsString()
+  @IsNotEmpty()
+  connectionId!: string;
+
+  @IsString()
+  @IsNotEmpty()
+  eventType!: string;
+}
+
+/**
+ * DTO for publishing an event
+ */
+export class PublishEventDto {
+  @IsString()
+  @IsNotEmpty()
+  eventType!: string;
+
+  @IsObject()
+  @IsNotEmpty()
+  payload!: Record<string, unknown>;
+}
+
+/**
+ * DTO for incoming event request
+ */
+export class IncomingEventDto {
+  @IsString()
+  @IsNotEmpty()
+  messageId!: string;
+
+  @IsString()
+  @IsNotEmpty()
+  instanceId!: string;
+
+  @IsString()
+  @IsNotEmpty()
+  eventType!: string;
+
+  @IsObject()
+  @IsNotEmpty()
+  payload!: Record<string, unknown>;
+
+  @IsNotEmpty()
+  timestamp!: number;
+
+  @IsString()
+  @IsNotEmpty()
+  signature!: string;
+}
+
+/**
+ * DTO for incoming event acknowledgment
+ */
+export class IncomingEventAckDto {
+  @IsString()
+  @IsNotEmpty()
+  messageId!: string;
+
+  @IsString()
+  @IsNotEmpty()
+  correlationId!: string;
+
+  @IsString()
+  @IsNotEmpty()
+  instanceId!: string;
+
+  @IsNotEmpty()
+  received!: boolean;
+
+  @IsOptional()
+  @IsString()
+  error?: string;
+
+  @IsNotEmpty()
+  timestamp!: number;
+
+  @IsString()
+  @IsNotEmpty()
+  signature!: string;
+}