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/adapters/store/channel_caption.go

@@ -0,0 +1,83 @@
+package store
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"github.com/jackc/pgx/v5"
+)
+
+// CaptionlessChannels returns the set of channel ids currently suppressed for the
+// user — channels whose recent videos all yielded no captions, within their
+// suppression window (ADR-024). The runner skips caption fetches for these
+// channels' videos. A channel whose window has expired is not returned, so its
+// next video is re-probed (auto-recovery).
+func (s *Store) CaptionlessChannels(ctx context.Context, userID string) (map[string]bool, error) {
+	out := map[string]bool{}
+	err := s.withUser(ctx, userID, func(tx pgx.Tx) error {
+		rows, err := tx.Query(ctx, `
+			SELECT channel_id FROM channel_caption_state
+			WHERE user_id = $1 AND captionless_until IS NOT NULL AND captionless_until > now()`,
+			userID)
+		if err != nil {
+			return fmt.Errorf("store: caption-less channels: %w", err)
+		}
+		defer rows.Close()
+		for rows.Next() {
+			var ch string
+			if err := rows.Scan(&ch); err != nil {
+				return fmt.Errorf("store: scan caption-less channel: %w", err)
+			}
+			out[ch] = true
+		}
+		return rows.Err()
+	})
+	return out, err
+}
+
+// RecordChannelCaptionOutcome updates a channel's caption-availability memory
+// after a fetch attempt (ADR-024). hadCaptions resets the channel (consecutive
+// count to 0, suppression cleared). Otherwise the consecutive no-caption count is
+// incremented; once it reaches threshold the channel is suppressed for window.
+// threshold <= 0 is a no-op (feature disabled). An empty channelID is ignored
+// (some sources may not carry one).
+func (s *Store) RecordChannelCaptionOutcome(ctx context.Context, userID, channelID string, hadCaptions bool, threshold int, window time.Duration) error {
+	if channelID == "" || threshold <= 0 {
+		return nil
+	}
+	return s.withUser(ctx, userID, func(tx pgx.Tx) error {
+		if hadCaptions {
+			_, err := tx.Exec(ctx, `
+				INSERT INTO channel_caption_state (user_id, channel_id, consecutive_none, captionless_until, updated_at)
+				VALUES ($1, $2, 0, NULL, now())
+				ON CONFLICT (user_id, channel_id)
+				DO UPDATE SET consecutive_none = 0, captionless_until = NULL, updated_at = now()`,
+				userID, channelID)
+			if err != nil {
+				return fmt.Errorf("store: reset channel caption state: %w", err)
+			}
+			return nil
+		}
+		// No captions: increment the streak; suppress once it reaches threshold.
+		// captionless_until is set from the NEW count inside the same statement so
+		// the decision is atomic with the increment.
+		until := time.Now().Add(window)
+		_, err := tx.Exec(ctx, `
+			INSERT INTO channel_caption_state (user_id, channel_id, consecutive_none, captionless_until, updated_at)
+			VALUES ($1, $2, 1, CASE WHEN 1 >= $3 THEN $4::timestamptz ELSE NULL END, now())
+			ON CONFLICT (user_id, channel_id)
+			DO UPDATE SET
+				consecutive_none = channel_caption_state.consecutive_none + 1,
+				captionless_until = CASE
+					WHEN channel_caption_state.consecutive_none + 1 >= $3 THEN $4::timestamptz
+					ELSE channel_caption_state.captionless_until
+				END,
+				updated_at = now()`,
+			userID, channelID, threshold, until)
+		if err != nil {
+			return fmt.Errorf("store: record channel no-caption: %w", err)
+		}
+		return nil
+	})
+}