feat(runner): TAPIR_FETCH_DELAY to throttle transcript fetches

Diagnosis of a live run: caption tracks resolve fine (player + watch-page scrape),
but the timedtext baseUrl fetch returns 429 under back-to-back volume — YouTube
rate-limits the unauthenticated caption-download endpoint per IP. A per-video
delay spaces the fetches. (Follow-up: treat 429 distinctly from genuine
no-caption instead of silently degrading to SourceNone; consider Whisper if the
endpoint stays hostile at any sustainable rate.)

internal/runner/runner.go

@@ -14,6 +14,7 @@ import (
 	"errors"
 	"fmt"
 	"log/slog"
+	"os"
 	"time"
 
 	"gitea.d-ma.be/mathias/tapir/internal/domain"
@@ -70,6 +71,12 @@ func (r *Runner) RunOnce(ctx context.Context) (Stats, error) {
 		errs  []error
 	)
 
+	// Throttle transcript fetches: unauthenticated caption scraping gets
+	// soft-throttled by YouTube under heavy back-to-back volume (captionTracks
+	// silently stripped from the player response). A small per-video delay keeps
+	// a full pass under the radar. TAPIR_FETCH_DELAY (Go duration), 0 = off.
+	fetchDelay, _ := time.ParseDuration(os.Getenv("TAPIR_FETCH_DELAY"))
+
 	seen, err := r.store.SeenVideoIDs(ctx, r.userID)
 	if err != nil {
 		return stats, fmt.Errorf("runner: load seen videos: %w", err)
@@ -105,6 +112,9 @@ func (r *Runner) RunOnce(ctx context.Context) (Stats, error) {
 			}
 			seen[id] = true // also guard against the same video within this pass
 
+			if fetchDelay > 0 {
+				time.Sleep(fetchDelay)
+			}
 			res, err := r.engine.ProcessNewVideo(ctx, v)
 			if err != nil {
 				errs = append(errs, fmt.Errorf("process %q: %w", v.ProviderVideoID, err))