Never show ideas for a failed ideas job, however it failed

A failed ideas_from_ingredients job could still show ideas through
GET /api/v1/jobs/{id}: a job failed at boot as interrupted by restart
keeps its last partial result, and when the write clearing the ideas
itself failed the runner recorded internal_error over the uncleared
result. Neither path can be fixed by a write.

The jobs package gains an opt-in per-kind hook, Kind.FailedResult,
which Runner.Get applies to a failed job's stored result on every read.
The ideas kind registers withoutIdeas, which empties the ideas and keeps
the rest of the result. Kinds that do not call FailedResult are
unchanged. The write-side clear stays as a tidy-up of what is stored.

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

internal/jobs/jobs_test.go

@@ -772,3 +772,47 @@ func TestServiceOnlyKind(t *testing.T) {
 		t.Error("ServiceOnly after Start succeeded, want an error")
 	}
 }
+
+func TestFailedResultAppliesOnlyToItsKindsFailedJobs(t *testing.T) {
+	t.Parallel()
+	st := migratedStore(t)
+	r := newRunner(t, st, 1)
+	h := func(_ context.Context, p countPayload, publish func(countResult) error) (countResult, error) {
+		if err := publish(countResult{Done: 5}); err != nil {
+			return countResult{}, err
+		}
+		if p.Steps == 0 {
+			return countResult{}, &Failure{Code: "stopped"}
+		}
+		return countResult{Done: p.Steps}, nil
+	}
+	hidden := register(t, r, "hidden", h, "stopped")
+	plain := register(t, r, "plain", h, "stopped")
+	if err := hidden.FailedResult(func(json.RawMessage) (json.RawMessage, error) {
+		return json.RawMessage(`{"done":0}`), nil
+	}); err != nil {
+		t.Fatal(err)
+	}
+	if err := hidden.FailedResult(nil); err == nil {
+		t.Error("FailedResult(nil) succeeded, want an error")
+	}
+	start(t, r)
+	if err := hidden.FailedResult(func(s json.RawMessage) (json.RawMessage, error) { return s, nil }); err == nil {
+		t.Error("FailedResult after Start succeeded, want an error")
+	}
+
+	for _, tc := range []struct {
+		kind  *Kind[countPayload]
+		steps int
+		want  string
+	}{
+		{hidden, 0, `{"done":0}`}, // failed: shown through FailedResult
+		{hidden, 3, `{"done":3}`}, // succeeded: as stored
+		{plain, 0, `{"done":5}`},  // failed, kind without FailedResult: as stored
+	} {
+		job := waitFor(t, r, enqueue(t, tc.kind, countPayload{Steps: tc.steps}), finished)
+		if string(job.Result) != tc.want {
+			t.Errorf("%s job with %d steps: %s result = %s, want %s", job.Kind, tc.steps, job.Status, job.Result, tc.want)
+		}
+	}
+}