feat: Initial MVP implementation

Summary

Initial implementation of Assemble - a membership management system for associations.

Features

• Member management (create, retrieve, list)
• Membership type management
• RESTful API with Actix-web
• PostgreSQL database with SQLx
• Docker support with multi-stage builds
• Dev container configuration

Documentation

• Complete requirements specification
• System design document
• Implementation task list

All MVP Phase 1 tasks completed.