feat: add devenv up support for local relay development
Adds processes.relay to devenv.nix so `devenv up` launches the relay via
cargo run, with EZPDS_* env vars providing dev defaults for data_dir and
public_url. relay.dev.toml holds committed non-secret defaults; state lands
in .devenv/state/relay (already gitignored). Override per-machine in
devenv.local.nix.