Refuse draft_week jobs on the generic jobs endpoint

A kind can now be made service-only: Runner.Enqueue, and so
POST /api/v1/jobs, refuses it with 400 service_only_kind, and only its
typed Kind enqueues it. draft_week is service-only, so StartDraft and
its one-draft-in-flight-per-week rule is the only way to start a draft;
a confirmed draft_week posted to /api/v1/jobs can no longer run beside
the week's unconfirmed draft.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/plan/plan.go

@@ -129,7 +129,8 @@ type Service struct {
 
 // NewService returns a service over st that drafts with draft and
 // registers the KindDraftWeek job on runner, which must not have started
-// yet.
+// yet. The kind is service-only: StartDraft, which keeps one draft in
+// flight per week, is the only way to enqueue it.
 func NewService(st *store.Store, draft Drafter, runner *jobs.Runner, logger *slog.Logger) (*Service, error) {
 	switch {
 	case st == nil:
@@ -146,6 +147,9 @@ func NewService(st *store.Store, draft Drafter, runner *jobs.Runner, logger *slo
 	if err != nil {
 		return nil, err
 	}
+	if err := kind.ServiceOnly(); err != nil {
+		return nil, err
+	}
 	s.draftJob = kind
 	return s, nil
 }