feat(runner): 429 backoff — skip still-throttled videos, persist status

After ProcessNewVideo the runner records transcript_status per outcome:
rate_limited (stamps the backoff clock), none, or fetched. Before fetching, a
video inside the TAPIR_FETCH_BACKOFF window is skipped (SkippedRateLimited) so a
just-429'd caption endpoint is not re-hit; once the window expires it retries.

Backoff/clock injected via variadic Options (WithBackoff, WithClock) so existing
New call sites and the fake-driven loop tests stay valid. Backoff 0 = always retry.

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

internal/runner/runner_test.go

@@ -5,6 +5,7 @@ import (
 	"io"
 	"log/slog"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/require"
 
@@ -43,11 +44,13 @@ func (f *fakeSource) FetchTranscript(_ context.Context, v domain.Video) (domain.
 // auto controls the summarization mode; requested is the manual-mode queue keyed
 // by store id; cleared records the ids whose queue flag the runner reset.
 type fakeStore struct {
-	seen      map[string]bool
-	upserted  []domain.Video
-	auto      bool
-	requested map[string]bool
-	cleared   []string
+	seen        map[string]bool
+	upserted    []domain.Video
+	auto        bool
+	requested   map[string]bool
+	cleared     []string
+	rateLimited map[string]time.Time // id -> when 429'd (seeds the backoff window)
+	statuses    map[string]string    // id -> last SetTranscriptStatus value
 }
 
 func (f *fakeStore) UpsertVideo(_ context.Context, v domain.Video) (string, error) {
@@ -80,6 +83,22 @@ func (f *fakeStore) ClearSummarizeRequested(_ context.Context, _, videoID string
 	return nil
 }
 
+func (f *fakeStore) RateLimitedVideoIDs(_ context.Context, _ string) (map[string]time.Time, error) {
+	cp := make(map[string]time.Time, len(f.rateLimited))
+	for k, v := range f.rateLimited {
+		cp[k] = v
+	}
+	return cp, nil
+}
+
+func (f *fakeStore) SetTranscriptStatus(_ context.Context, _, videoID, status string) error {
+	if f.statuses == nil {
+		f.statuses = map[string]string{}
+	}
+	f.statuses[videoID] = status
+	return nil
+}
+
 type fakeSummarizer struct{}
 
 func (fakeSummarizer) Summarize(_ context.Context, v domain.Video, _ domain.Transcript) (domain.Summary, error) {
@@ -207,6 +226,62 @@ func TestRunOnce_ManualMode_ProcessesRequested(t *testing.T) {
 	require.Equal(t, []string{"id-v1"}, st.cleared, "the queue flag is cleared after summarizing")
 }
 
+// noFetchSource fails the test if a transcript fetch happens — used to prove the
+// runner skips a rate-limited video before touching the caption endpoint.
+type noFetchSource struct{ *fakeSource }
+
+func (noFetchSource) FetchTranscript(context.Context, domain.Video) (domain.Transcript, error) {
+	panic("FetchTranscript must not be called for a rate-limited video within the backoff window")
+}
+
+func TestRunOnce_SkipsRateLimitedWithinBackoff(t *testing.T) {
+	base := time.Date(2026, 6, 3, 12, 0, 0, 0, time.UTC)
+	src := &fakeSource{
+		subs:   []domain.Subscription{sub("chan1", "Channel One")},
+		videos: map[string][]domain.Video{"chan1": {vid("v1", "Video 1")}},
+	}
+	// v1 was rate-limited 5m ago; backoff is 1h, so it is still inside the window.
+	st := &fakeStore{
+		seen:        map[string]bool{},
+		auto:        true,
+		rateLimited: map[string]time.Time{"id-v1": base.Add(-5 * time.Minute)},
+	}
+	eng := usecase.NewEngine(noFetchSource{src}, fakeSummarizer{}, &recordingSink{})
+	r := runner.New(noFetchSource{src}, st, eng, testUser, quietLogger(),
+		runner.WithBackoff(time.Hour), runner.WithClock(func() time.Time { return base }))
+
+	stats, err := r.RunOnce(context.Background())
+	require.NoError(t, err)
+	require.Equal(t, 1, stats.SkippedRateLimited, "still throttled -> skipped")
+	require.Equal(t, 0, stats.Summarized)
+	require.Empty(t, st.statuses, "no status write: the engine was never invoked")
+}
+
+func TestRunOnce_RetriesRateLimitedAfterBackoff(t *testing.T) {
+	base := time.Date(2026, 6, 3, 12, 0, 0, 0, time.UTC)
+	src := &fakeSource{
+		subs:   []domain.Subscription{sub("chan1", "Channel One")},
+		videos: map[string][]domain.Video{"chan1": {vid("v1", "Video 1")}},
+	}
+	// v1 was rate-limited 2h ago; backoff is 1h, so the window has expired.
+	st := &fakeStore{
+		seen:        map[string]bool{},
+		auto:        true,
+		rateLimited: map[string]time.Time{"id-v1": base.Add(-2 * time.Hour)},
+	}
+	sink := &recordingSink{}
+	eng := usecase.NewEngine(src, fakeSummarizer{}, sink)
+	r := runner.New(src, st, eng, testUser, quietLogger(),
+		runner.WithBackoff(time.Hour), runner.WithClock(func() time.Time { return base }))
+
+	stats, err := r.RunOnce(context.Background())
+	require.NoError(t, err)
+	require.Equal(t, 0, stats.SkippedRateLimited, "window expired -> not skipped")
+	require.Equal(t, 1, stats.Summarized, "the video is retried and summarized")
+	require.Len(t, sink.delivered, 1)
+	require.Equal(t, "fetched", st.statuses["id-v1"], "status advances to fetched on success")
+}
+
 func TestRunOnce_UpsertsEveryCandidate(t *testing.T) {
 	src := &fakeSource{
 		subs:   []domain.Subscription{sub("chan1", "Channel One")},