Add Railway deployment config

[arferreira]

Summary

• Multi-stage Dockerfile that builds the dashboard WASM (trunk) before compiling the backend binary (rust-embed bakes dist/ into the binary)
• Rust version bumped from 1.85 to 1.88 to match CI
• New railway.toml for Dockerfile-based builds with health checks on /health

Manual steps after merge

1. Create Railway project, add PostgreSQL addon
2. Connect zaptech-dev/reeverb repo (main branch)
3. Set env vars: JWT_SECRET, RUST_LOG=info, PORT=3000
4. DATABASE_URL auto-injected from Postgres addon

Test plan

• docker build -t reeverb . builds successfully
• Container starts, /health returns 200
• Railway deploys on push to main