Public signup configuration

Returns the per-host signup mode (`create_tenant` or `join_tenant`), whether signup is enabled, and — when the requesting host resolves to an existing tenant — that tenant's display name + slug. The mode is resolved per-request from the request host (tenant_hostnames), not from a deploy-time setting. The SPA's `/signup` form uses this to adapt its UI: in `join_tenant` mode it hides tenant_name + tenant_slug fields and surfaces "Joining {tenant_name}". An unrecognized host reports `create_tenant` with no tenant fields. Public, no auth, no rate limit.

curl -X GET "https://example.com/v1/auth/signup/config"

{
  "mode": "join_tenant",
  "signup_enabled": true,
  "tenant_name": "Acme Corp",
  "tenant_slug": "acme-corp"
}