feat(config): TAPIR_FETCH_BACKOFF for rate-limit retry window

Adds FetchBackoff (Go duration, default 1h) controlling how long the run loop
waits before re-fetching a transcript that returned HTTP 429. Zero means always
retry. Not required by ValidateForRun — a zero/unset value is a valid policy.

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

internal/config/config.go

@@ -59,6 +59,12 @@ type Config struct {
 	// PollInterval, when > 0, makes `run` loop on that cadence; 0 means run once.
 	PollInterval time.Duration
 
+	// FetchBackoff is how long the run loop waits before re-fetching a transcript
+	// that previously returned HTTP 429 (rate_limited). Inside the window the video
+	// is skipped without hitting the caption endpoint, saving requests; after it
+	// expires the video is retried. Zero means "always retry" (no backoff).
+	FetchBackoff time.Duration
+
 	// HTTPAddr is the listen address for `tapir serve` (the Stage-0 web UI).
 	HTTPAddr string
 
@@ -85,6 +91,7 @@ const (
 	defaultYTConnectRedirectURL = "https://tapir.d-ma.be/oauth/youtube/callback"
 	defaultOAuthRedirectAddr    = "localhost:8080"
 	defaultHTTPAddr             = ":8080"
+	defaultFetchBackoff         = time.Hour
 )
 
 // Load reads the environment into a Config, applying defaults. It does not
@@ -124,6 +131,12 @@ func Load() (Config, error) {
 	}
 	c.PollInterval = interval
 
+	backoff, err := durationOr("TAPIR_FETCH_BACKOFF", defaultFetchBackoff)
+	if err != nil {
+		return Config{}, err
+	}
+	c.FetchBackoff = backoff
+
 	return c, nil
 }