feat(MM-69): configuration system — relay.toml parsing · pull #8

Summary#

Adds Config struct to the common crate (serde-deserializable from TOML) with v0.1 fields: bind_address, port, data_dir,database_url, public_url, and empty stub sections [blobs], [oauth], [iroh]
EZPDS_* env var overrides (prefix: EZPDS_) applied on top of TOML via pure apply_env_overrides — no I/O in the Functional Core
Relay binary gains --config/EZPDS_CONFIG CLI arg (clap), loads config on startup, fails fast with clear error message on invalid config; logging via RUST_LOG-aware EnvFilter
13 tests: TOML parsing, env var overrides, missing required field errors, I/O and parse error paths

FCIS pattern applied:

config.rs — Functional Core: Config, RawConfig, ConfigError, apply_env_overrides (takes explicit env HashMap), validate_and_build
config_loader.rs — Imperative Shell: load_config (reads file + real env), load_config_with_env (pub(crate) for test isolation)
relay/src/main.rs — Imperative Shell: CLI parsing, config loading, structured logging

cargo test --workspace — 13 tests pass, 0 failures
cargo clippy --workspace -- -D warnings — no warnings
cargo fmt --all --check — clean
cargo build --package relay — builds successfully
Manual smoke test: run ./target/debug/relay --config /nonexistent.toml and confirm error message is error: failed to load config from /nonexistent.toml: failed to read config file: No such file or directory (os error 2)
Manual smoke test: create a minimal relay.toml with data_dir and public_url, run ./target/debug/relay and confirm structured log line at startup

Closes MM-69