Default Mailglass.Tenancy resolver: scope/2 is a no-op;
resolve_webhook_tenant/1 returns {:ok, "default"}.
Single-tenant adopters need zero configuration — this is the
Mailglass.Config-resolved default when :tenancy is nil.
Mailglass.Tenancy.current/0 returns the literal string "default"
when this resolver is active and no explicit put_current/1 has
run, and webhook ingest (Phase 4) stamps the same "default" value
on every verified request.
Adopters who want real tenant isolation implement their own module
with @behaviour Mailglass.Tenancy and wire it via:
config :mailglass, tenancy: MyApp.TenancyMulti-tenant adopters override resolve_webhook_tenant/1 to map
verified webhook contexts (headers / path_params / conn / raw_body)
to tenant_ids. See the Mailglass.Tenancy moduledoc for the
callback's 6-key context map shape and examples.