fix(#338): Validate DEFAULT_WORKSPACE_ID as UUID

- Add federation.config.ts with UUID v4 validation for DEFAULT_WORKSPACE_ID
- Validate at module initialization (fail fast if misconfigured)
- Replace hardcoded "default" fallback with proper validation
- Add 18 tests covering valid UUIDs, invalid formats, and missing values
- Clear error messages with expected UUID format

Refs #338

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

apps/api/src/federation/federation.module.ts

@@ -3,9 +3,10 @@
  *
  * Provides instance identity and federation management with DoS protection via rate limiting.
  * Issue #272: Rate limiting added to prevent DoS attacks on federation endpoints
+ * Issue #338: Validate DEFAULT_WORKSPACE_ID at startup
  */
 
-import { Module } from "@nestjs/common";
+import { Module, Logger, OnModuleInit } from "@nestjs/common";
 import { ConfigModule } from "@nestjs/config";
 import { HttpModule } from "@nestjs/axios";
 import { ThrottlerModule } from "@nestjs/throttler";
@@ -20,6 +21,7 @@ import { OIDCService } from "./oidc.service";
 import { CommandService } from "./command.service";
 import { QueryService } from "./query.service";
 import { FederationAgentService } from "./federation-agent.service";
+import { validateFederationConfig } from "./federation.config";
 import { PrismaModule } from "../prisma/prisma.module";
 import { TasksModule } from "../tasks/tasks.module";
 import { EventsModule } from "../events/events.module";
@@ -83,4 +85,22 @@ import { RedisProvider } from "../common/providers/redis.provider";
     FederationAgentService,
   ],
 })
-export class FederationModule {}
+export class FederationModule implements OnModuleInit {
+  private readonly logger = new Logger(FederationModule.name);
+
+  /**
+   * Validate federation configuration at module initialization.
+   * Issue #338: Fail fast if DEFAULT_WORKSPACE_ID is not a valid UUID.
+   */
+  onModuleInit(): void {
+    try {
+      validateFederationConfig();
+      this.logger.log("Federation configuration validated successfully");
+    } catch (error) {
+      this.logger.error(
+        `Federation configuration validation failed: ${error instanceof Error ? error.message : String(error)}`
+      );
+      throw error;
+    }
+  }
+}