feat(config): support TOML config file alongside env vars

Summary

• Add TOML config file support: loads /app/config.toml (container) or ./config.toml (local dev) when present
• Env vars override file values; missing file silently ignored so env-only deployments keep working
• Unknown keys and malformed TOML fail fast before polling begins
• Dockerfile copies config.toml to /app/config.toml as the default container config

Test plan

• make py/pytest passes (49 tests)
• make run connects to meter and emits metric_read_succeeded events
• Start with config file only (no env vars) — service starts normally
• Start with env var overriding a file value — env var wins
• Start with malformed TOML — fails before first poll with configuration_error event
• Start with unknown TOML key — fails with key name in error message