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/federation/federation.module.ts

@@ -0,0 +1,19 @@
+/**
+ * Federation Module
+ *
+ * Provides instance identity and federation management.
+ */
+
+import { Module } from "@nestjs/common";
+import { ConfigModule } from "@nestjs/config";
+import { FederationController } from "./federation.controller";
+import { FederationService } from "./federation.service";
+import { PrismaModule } from "../prisma/prisma.module";
+
+@Module({
+  imports: [ConfigModule, PrismaModule],
+  controllers: [FederationController],
+  providers: [FederationService],
+  exports: [FederationService],
+})
+export class FederationModule {}