feat(#84): implement instance identity model for federation

Implemented the foundation of federation architecture with instance
identity and connection management:

Database Schema:
- Added Instance model for instance identity with keypair generation
- Added FederationConnection model for workspace-scoped connections
- Added FederationConnectionStatus enum (PENDING, ACTIVE, SUSPENDED, DISCONNECTED)

Service Layer:
- FederationService with instance identity management
- RSA 2048-bit keypair generation for signing
- Public identity endpoint (excludes private key)
- Keypair regeneration capability

API Endpoints:
- GET /api/v1/federation/instance - Returns public instance identity
- POST /api/v1/federation/instance/regenerate-keys - Admin keypair regeneration

Tests:
- 11 tests passing (7 service, 4 controller)
- 100% statement coverage, 100% function coverage
- Follows TDD principles (Red-Green-Refactor)

Configuration:
- Added INSTANCE_NAME and INSTANCE_URL environment variables
- Integrated FederationModule into AppModule

Refs #84

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

apps/api/src/app.module.ts

@@ -31,6 +31,7 @@ import { RunnerJobsModule } from "./runner-jobs/runner-jobs.module";
 import { JobEventsModule } from "./job-events/job-events.module";
 import { JobStepsModule } from "./job-steps/job-steps.module";
 import { CoordinatorIntegrationModule } from "./coordinator-integration/coordinator-integration.module";
+import { FederationModule } from "./federation/federation.module";
 
 @Module({
   imports: [
@@ -84,6 +85,7 @@ import { CoordinatorIntegrationModule } from "./coordinator-integration/coordina
     JobEventsModule,
     JobStepsModule,
     CoordinatorIntegrationModule,
+    FederationModule,
   ],
   controllers: [AppController],
   providers: [