Add the AGPL-3.0 license (matching Orbit) and set it in Cargo.toml so
the crate metadata is complete. Handle SIGTERM/Ctrl-C: the run loop now
selects against a shutdown signal and exits cleanly, so systemctl stop
returns promptly instead of waiting out the kill timeout (every event is
already fsync'd, so nothing is lost either way). Verified: on SIGTERM the
daemon logs and exits 0.
