/sync/{athlete_id} always rendered 'Sync: <session.display_name>',
even when athlete_id (now possibly a foreign athlete, see the
previous 'allow syncing athletes other than the current session'
change) differed from the session's own athlete. The header now
Sessions previously had no expiry at all: a county_session cookie
was valid forever until an explicit /logout. Both session lookups
now reject rows older than 1 day (created_at-based, not sliding),
and db::delete_expired_sessions prunes expired rows so the table
doesn't grow unbounded; it's called during the existing OAuth-start
housekeeping alongside the oauth_states cleanup.
db::ensure_dev_athlete was never called (main.rs had its own,
inferior copy, removed in the previous commit). Config::dev_sync_days
and Config::require_api_key were only used by the now-removed
/dev/sync handlers.
