Scaffold initial backend and PWA

[leynos]

Summary

• add Actix backend with user API, WebSocket stub, and OpenAPI wiring
• scaffold React PWA, design tokens package, and API client fetcher
• include Docker, Compose, and Kubernetes deployment manifests with CI pipeline
• align AsyncAPI channel with WebSocket route and expand payload schema
• replace empty OpenAPI placeholder with minimal user endpoint spec
• harden design token build script and format theme definitions
• remove Bun lockfile from repository and ignore it

Testing

• make fmt
• make lint
• make test
• bun audit (fails: Script not found "audit")

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https://chatgpt.com/codex/tasks/task_e_689c4491c2248322afe54448c92187a7