Agent registration validated web_server against nginx|apache|none only, so
agents behind Caddy/FrankenPHP or Traefik (e.g. bale-cms / dindik-profile)
were rejected on register even though the agent binary supports those
collectors. Widen the enum to nginx,apache,caddy,frankenphp,traefik,none.
The DB column is string(16), so no migration is needed. Pairs with
nawasara/agent v0.9.0 (Traefik collector).