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>

cmd/tapir/main.go

@@ -136,7 +136,8 @@ func cmdRun(ctx context.Context, log *slog.Logger) error {
 
 	r := runner.New(engine.Source, st, engine, cfg.UserID, log,
 		runner.WithBackoff(cfg.FetchBackoff),
-		runner.WithAutoWindow(cfg.AutoSummarizeWindow))
+		runner.WithAutoWindow(cfg.AutoSummarizeWindow),
+		runner.WithCaptionMemory(cfg.ChannelCaptionlessThreshold, cfg.ChannelCaptionlessWindow))
 
 	log.Info("starting run", "user", cfg.UserID, "model", cfg.SummarizerModel,
 		"gateway", cfg.GatewayURL, "poll_interval", cfg.PollInterval, "fetch_backoff", cfg.FetchBackoff,