Refactor customer → instance package, add dock service, update provision.sh

Refactor customer → instance package, add dock service, update provision.sh

- Rename internal/customer → internal/instance, customers table → instances
- Rename API endpoint /customers → /instances
- Add internal/instance package with clean schema (drops email, subscription ID)
- Add JSON struct tags, exclude ssh_private_key from responses
- Build out crimata-dock C service (libmicrohttpd + libsystemd)
- Update provision.sh to install full OS stack from adgundersen/os
- Add MIT License

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>