feat: scaffold digital asset management service

[wumibals]

Summary

Initializes the asset management service microservice under microservices/asset-service with NestJS, TypeORM entities, service layer, and REST controller for managing uploaded files and digital assets.

Changes

• Add AppModule with PostgreSQL/TypeORM configuration
• Add Asset entity with owner, mime type, size, and soft-delete status
• Add AssetService with register, lookup, and soft-delete operations
• Add AssetController exposing REST endpoints
• Add main.ts entry point

closes #317

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