fix(#85): resolve TypeScript compilation and validation issues

- Fix @IsNumber() validator on timestamp field (was @IsString() - critical security issue)
- Fix TypeScript compilation error in sortObjectKeys array handling
- Replace generic Error with UnauthorizedException and ServiceUnavailableException
- Document hardcoded workspace ID limitation in handleIncomingConnection
- Remove unused BadRequestException import

All tests passing (70/70), TypeScript compiles cleanly, linting passes.

apps/api/src/federation/federation.controller.ts

@@ -195,8 +195,10 @@ export class FederationController {
   ): Promise<{ status: string; connectionId?: string }> {
     this.logger.log(`Received connection request from ${dto.instanceId}`);
 
-    // For now, create connection in default workspace
-    // TODO: Allow configuration of which workspace handles incoming connections
+    // LIMITATION: Incoming connections are created in a default workspace
+    // TODO: Future enhancement - Allow configuration of which workspace handles incoming connections
+    // This could be based on routing rules, instance configuration, or a dedicated federation workspace
+    // For now, uses DEFAULT_WORKSPACE_ID environment variable or falls back to "default"
     const workspaceId = process.env.DEFAULT_WORKSPACE_ID ?? "default";
 
     const connection = await this.connectionService.handleIncomingConnectionRequest(