feat(discovery): per-channel caption-availability memory (ADR-024)

After transcript caching (ADR-021) and the Shorts filter (ADR-023), the remaining
caption waste is the first fetch on every new video of a channel that never has
English captions — each costs one rate-limited fetch to resolve to "none", and on
a throttled IP churns the backoff machinery first.

Remember, per (user, channel), a streak of consecutive no-caption outcomes
(channel_caption_state, migration 016, RLS-scoped). Once it reaches
TAPIR_CHANNEL_CAPTIONLESS_THRESHOLD (default 5) the channel is suppressed — videos
discovered/listed but not caption-fetched — for TAPIR_CHANNEL_CAPTIONLESS_WINDOW
(default 14d), then one is re-probed (auto-recovery). A successful fetch resets
the streak; a 429 does not count; an explicit manual request bypasses suppression.
threshold=0 disables.

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

internal/runner/runner_test.go

@@ -51,6 +51,13 @@ type fakeStore struct {
 	cleared     []string
 	rateLimited map[string]time.Time // id -> when 429'd (seeds the backoff window)
 	statuses    map[string]string    // id -> last SetTranscriptStatus value
+	captionless map[string]bool      // channel ids currently suppressed (ADR-024)
+	captionRecs []captionRec         // RecordChannelCaptionOutcome calls, in order
+}
+
+type captionRec struct {
+	channelID string
+	had       bool
 }
 
 func (f *fakeStore) UpsertVideo(_ context.Context, v domain.Video) (string, error) {
@@ -93,6 +100,22 @@ func (f *fakeStore) RateLimitedVideoIDs(_ context.Context, _ string) (map[string
 
 func (f *fakeStore) UpsertChannelError(_ context.Context, _, _, _ string) error { return nil }
 
+func (f *fakeStore) CaptionlessChannels(_ context.Context, _ string) (map[string]bool, error) {
+	cp := make(map[string]bool, len(f.captionless))
+	for k, v := range f.captionless {
+		cp[k] = v
+	}
+	return cp, nil
+}
+
+func (f *fakeStore) RecordChannelCaptionOutcome(_ context.Context, _, channelID string, hadCaptions bool, threshold int, _ time.Duration) error {
+	if threshold <= 0 {
+		return nil
+	}
+	f.captionRecs = append(f.captionRecs, captionRec{channelID: channelID, had: hadCaptions})
+	return nil
+}
+
 func (f *fakeStore) SetTranscriptStatus(_ context.Context, _, videoID, status string) error {
 	if f.statuses == nil {
 		f.statuses = map[string]string{}
@@ -284,6 +307,55 @@ func TestRunOnce_AutoMode_OldVideoRequestedBypassesWindow(t *testing.T) {
 	require.Len(t, sink.delivered, 1)
 }
 
+// TestRunOnce_CaptionlessChannelSkipped: a channel flagged caption-less (ADR-024)
+// has its videos skipped from fetching but still discovered/listed, while a
+// normal channel's video is summarized.
+func TestRunOnce_CaptionlessChannelSkipped(t *testing.T) {
+	src := &fakeSource{
+		subs: []domain.Subscription{sub("dead", "Dead Channel"), sub("live", "Live Channel")},
+		videos: map[string][]domain.Video{
+			"dead": {vid("d1", "Dead One")},
+			"live": {vid("l1", "Live One")},
+		},
+	}
+	st := &fakeStore{seen: map[string]bool{}, auto: true, captionless: map[string]bool{"dead": true}}
+	sink := &recordingSink{}
+	eng := usecase.NewEngine(src, fakeSummarizer{}, sink)
+	r := runner.New(src, st, eng, testUser, quietLogger(),
+		runner.WithCaptionMemory(5, 14*24*time.Hour))
+
+	stats, err := r.RunOnce(context.Background())
+	require.NoError(t, err)
+	require.Equal(t, 1, stats.SkippedNoCaptionChannel, "dead channel's video skipped from fetch")
+	require.Equal(t, 1, stats.Summarized, "live channel's video still summarized")
+	require.Len(t, st.upserted, 2, "both videos are still discovered and listed")
+}
+
+// TestRunOnce_RecordsCaptionOutcomes: a no-caption result grows the channel's
+// streak (had=false); a successful summary resets it (had=true).
+func TestRunOnce_RecordsCaptionOutcomes(t *testing.T) {
+	src := &fakeSource{
+		subs: []domain.Subscription{sub("c1", "Has Caps"), sub("c2", "No Caps")},
+		videos: map[string][]domain.Video{
+			"c1": {vid("good", "Good")},
+			"c2": {vid("bad", "Bad")},
+		},
+		transcripts: map[string]domain.Transcript{
+			"bad": {Source: domain.SourceNone}, // no usable text → engine skips
+		},
+	}
+	st := &fakeStore{seen: map[string]bool{}, auto: true}
+	sink := &recordingSink{}
+	eng := usecase.NewEngine(src, fakeSummarizer{}, sink)
+	r := runner.New(src, st, eng, testUser, quietLogger(),
+		runner.WithCaptionMemory(5, 14*24*time.Hour))
+
+	_, err := r.RunOnce(context.Background())
+	require.NoError(t, err)
+	require.Contains(t, st.captionRecs, captionRec{channelID: "c1", had: true}, "captioned channel reset")
+	require.Contains(t, st.captionRecs, captionRec{channelID: "c2", had: false}, "no-caption channel streak grown")
+}
+
 // TestRunOnce_AutoWindowZero_SummarizesOld: a zero window disables the bound —
 // the pre-recency behaviour (summarize every unseen video) is preserved.
 func TestRunOnce_AutoWindowZero_SummarizesOld(t *testing.T) {