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/prisma/schema.prisma

@@ -268,9 +268,10 @@ model Workspace {
   personalities         Personality[]
   llmSettings           WorkspaceLlmSettings?
   qualityGates          QualityGate[]
-  runnerJobs            RunnerJob[]
-  federationConnections FederationConnection[]
-  federationMessages    FederationMessage[]
+  runnerJobs                    RunnerJob[]
+  federationConnections         FederationConnection[]
+  federationMessages            FederationMessage[]
+  federationEventSubscriptions  FederationEventSubscription[]
 
   @@index([ownerId])
   @@map("workspaces")
@@ -1287,8 +1288,9 @@ model FederationConnection {
   disconnectedAt DateTime? @map("disconnected_at") @db.Timestamptz
 
   // Relations
-  workspace Workspace           @relation(fields: [workspaceId], references: [id], onDelete: Cascade)
-  messages  FederationMessage[]
+  workspace        Workspace                     @relation(fields: [workspaceId], references: [id], onDelete: Cascade)
+  messages         FederationMessage[]
+  eventSubscriptions FederationEventSubscription[]
 
   @@unique([workspaceId, remoteInstanceId])
   @@index([workspaceId])
@@ -1330,6 +1332,7 @@ model FederationMessage {
   // Message content
   query       String? @db.Text
   commandType String? @map("command_type") @db.Text
+  eventType   String? @map("event_type") @db.Text // For EVENT messages
   payload     Json?   @default("{}")
   response    Json?   @default("{}")
 
@@ -1353,5 +1356,30 @@ model FederationMessage {
   @@index([connectionId])
   @@index([messageId])
   @@index([correlationId])
+  @@index([eventType])
   @@map("federation_messages")
 }
+
+model FederationEventSubscription {
+  id           String @id @default(uuid()) @db.Uuid
+  workspaceId  String @map("workspace_id") @db.Uuid
+  connectionId String @map("connection_id") @db.Uuid
+
+  // Event subscription details
+  eventType String   @map("event_type")
+  metadata  Json     @default("{}")
+  isActive  Boolean  @default(true) @map("is_active")
+  createdAt DateTime @default(now()) @map("created_at") @db.Timestamptz
+  updatedAt DateTime @updatedAt @map("updated_at") @db.Timestamptz
+
+  // Relations
+  connection FederationConnection @relation(fields: [connectionId], references: [id], onDelete: Cascade)
+  workspace  Workspace            @relation(fields: [workspaceId], references: [id], onDelete: Cascade)
+
+  @@unique([workspaceId, connectionId, eventType])
+  @@index([workspaceId])
+  @@index([connectionId])
+  @@index([eventType])
+  @@index([workspaceId, isActive])
+  @@map("federation_event_subscriptions")
+}