feat(youtube): map caption 429 to SourceRateLimited

The baseUrl fetch mapped every non-200 to SourceNone, recording a 429 as a
permanent "no captions". 429 is the IP being rate-limited, not an absent
transcript. Return SourceRateLimited (still a graceful degrade, no error) so
the runner can retry after a backoff window. Other non-200s (403/404/5xx)
stay SourceNone.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/adapters/youtube/youtube_test.go

@@ -388,6 +388,37 @@ func TestFetchTranscriptBaseURLForbiddenDegrades(t *testing.T) {
 	}
 }
 
+// A 429 on the baseUrl fetch is the IP being rate-limited, NOT a permanent
+// absence of captions: it returns SourceRateLimited (no error, no text) so the
+// runner can record it and retry after a backoff window rather than recording a
+// false "no transcript".
+func TestFetchTranscriptRateLimitedReturnsSourceRateLimited(t *testing.T) {
+	a, _ := newTestAdapter(t, func(w http.ResponseWriter, r *http.Request) {
+		switch r.URL.Path {
+		case "/youtubei/v1/player":
+			base := "http://" + r.Host
+			_, _ = w.Write([]byte(`{"captions":{"playerCaptionsTracklistRenderer":{"captionTracks":[` +
+				`{"baseUrl":"` + base + `/api/timedtext?lang=en","languageCode":"en"}]}}}`))
+		case "/api/timedtext":
+			w.WriteHeader(http.StatusTooManyRequests)
+		}
+	})
+
+	tr, err := a.FetchTranscript(context.Background(), domain.Video{ID: "v1", UserID: "u1", ProviderVideoID: "vid1"})
+	if err != nil {
+		t.Fatalf("429 on baseUrl must degrade, not error: %v", err)
+	}
+	if tr.Source != domain.SourceRateLimited {
+		t.Fatalf("expected SourceRateLimited on 429, got %q", tr.Source)
+	}
+	if tr.HasText() {
+		t.Error("expected HasText() false for SourceRateLimited")
+	}
+	if tr.Content != "" {
+		t.Errorf("expected empty content on 429, got %q", tr.Content)
+	}
+}
+
 // An empty baseUrl on the selected track degrades to SourceNone, never an error.
 func TestFetchTranscriptEmptyBaseURLDegrades(t *testing.T) {
 	a, _ := newTestAdapter(t, func(w http.ResponseWriter, r *http.Request) {