phraser: report the failure next to the fallback (V-397)

PhraseChat and PhraseQuery returned canned text with a nil error, so a dead
or OOM-killed server was indistinguishable from bad phrasing — "не знаю." is
also a legitimate answer.

Both now return the fallback text AND the error. The daemon keeps using the
text, so the turn still survives; a measuring caller counts a real failure.
An empty response is its own error: the model is up and said nothing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/phraser/swap_test.go

@@ -215,10 +215,12 @@ func TestSwap_RollbackFailureLeavesNoBackendAndDegrades(t *testing.T) {
 	if _, _, aerr := p.acquire(); !errors.Is(aerr, ErrNoBackend) {
 		t.Errorf("acquire error = %v; want ErrNoBackend", aerr)
 	}
-	// Phrasing degrades to its fallback instead of failing the turn.
+	// Phrasing degrades to its fallback instead of failing the turn, and since
+	// Vikunja #397 it reports the error next to that fallback so a measuring
+	// caller can tell "no model" from "bad phrasing".
 	got, err := p.PhraseChat(context.Background(), "привет", nil)
-	if err != nil {
-		t.Fatalf("PhraseChat after a total failure returned an error: %v", err)
+	if !errors.Is(err, ErrNoBackend) {
+		t.Errorf("PhraseChat error = %v; want ErrNoBackend alongside the fallback", err)
 	}
 	if got == "" {
 		t.Error("PhraseChat returned empty; the fallback must still say something")