Add architecture + design docs with domain model

[ric03uec]

Summary

Create architecture and design documentation for Clawrium, including a shared domain model to guide future features.

Goals

• Provide a clear mental model of the system
• Align future work with shared vocabulary and boundaries
• Reduce onboarding time for contributors

Requirements

• Architecture overview (components, data flow, deployment model)
• Domain model with key entities and relationships (Host, Claw, Registry, Instance, Fleet, etc.)
• Sequence diagrams for common flows (deploy, upgrade, backup, metrics collection)
• Data model section aligned with current storage (and future SQLite)
• Place docs under docs/ (or existing conventions)

Acceptance Criteria

• Docs are discoverable from README
• Domain model is referenced in new PRDs
• At least one diagram per major flow

Notes

• Keep diagrams text-based (Mermaid) for easy maintenance