Class AppModule

Root application module.

Module loading order:

1. AppConfigModule — global config and env validation (must be first).
2. BullModule.forRootAsync — sets up the shared BullMQ Redis connection.
3. GrpcModule — global gRPC client for the Rust controller engine.
4. MetadataModule — global metadata Redis store.
5. Feature modules (KeyGenerationModule, SigningModule, JobsModule).