Rewrite backend in Rust (FastAPI → Rust)

[moonexpr]

Goal

Rewrite the Python/FastAPI backend (backend/app) in Rust, motivating the move of this fork into the Monogarden (~/Garden/external/claude-deck) where it can later integrate with the harvest build system and shared Rust libs.

Context

• Forked from adrirubio/claude-deck @ 0ea9286 (v1.1.0).
• Currently deployed on moonray (FreeBSD home server) as the Python backend serving the built React frontend, behind Cloudflare Access at deck.moonexpr.com. Deployment is the interim state; the Rust rewrite is the end goal.
• Frontend (React 19 + Vite) stays as-is; this is a backend-only port.

Scope (high level — to be decomposed)

• Choose Rust web framework (axum/actix) + async SQLite (sqlx)
• Port /api/v1 routers: config, mcp, commands, plugins, hooks, permissions, agents, backup, sessions, usage, memory, cc-bridge, etc.
• Preserve the exact JSON contract the frontend expects (no frontend changes)
• Reproduce CC Bridge (tmux discovery + PTY relay) — the most involved subsystem
• Serve the built frontend/dist as static assets
• Maintain SQLite schema + WAL behavior
• Parity test suite vs the Python backend before cutover

Non-goals

• Frontend rewrite
• Behavioral changes / new features during the port

Acceptance

• Rust backend passes a request/response parity check against the Python backend for all /api/v1 endpoints
• CC Bridge attaches/relays a real tmux Claude session
• moonray redeployed on the Rust backend with no frontend changes

Deferred — tracked, not started this session.