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/server/jobs.go

@@ -30,7 +30,8 @@ type jobCreated struct {
 }
 
 // postJob serves POST /api/v1/jobs: it enqueues a job of a registered kind
-// and returns 202 with its id, which GET /api/v1/jobs/{id} reports on.
+// that is not service-only and returns 202 with its id, which
+// GET /api/v1/jobs/{id} reports on.
 func postJob(logger *slog.Logger, runner *jobs.Runner) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
 		req, err := decodeJobRequest(http.MaxBytesReader(w, r.Body, maxJobBody))
@@ -43,6 +44,9 @@ func postJob(logger *slog.Logger, runner *jobs.Runner) http.HandlerFunc {
 		case errors.Is(err, jobs.ErrUnknownKind):
 			writeError(w, r, logger, http.StatusBadRequest, "unknown_kind", "unknown job kind")
 			return
+		case errors.Is(err, jobs.ErrServiceOnly):
+			writeError(w, r, logger, http.StatusBadRequest, "service_only_kind", "job kind is started only by its own endpoint")
+			return
 		case errors.Is(err, jobs.ErrInvalidPayload):
 			writeError(w, r, logger, http.StatusBadRequest, "invalid_payload", "payload does not match the job kind")
 			return