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_test.go

@@ -45,6 +45,9 @@ func TestLoad_AppliesDefaults(t *testing.T) {
 	if c.PollInterval != 0 {
 		t.Errorf("PollInterval = %v, want 0 (run once)", c.PollInterval)
 	}
+	if c.FetchBackoff != defaultFetchBackoff {
+		t.Errorf("FetchBackoff = %v, want default %v", c.FetchBackoff, defaultFetchBackoff)
+	}
 }
 
 func TestLoad_ParsesValues(t *testing.T) {
@@ -56,6 +59,7 @@ func TestLoad_ParsesValues(t *testing.T) {
 		"TAPIR_SUMMARIZER_TIMEOUT": "90s",
 		"TAPIR_DB_DSN":             "postgres://x",
 		"TAPIR_POLL_INTERVAL":      "10m",
+		"TAPIR_FETCH_BACKOFF":      "30m",
 	})
 
 	c, err := Load()
@@ -77,6 +81,9 @@ func TestLoad_ParsesValues(t *testing.T) {
 	if c.PollInterval != 10*time.Minute {
 		t.Errorf("PollInterval = %v, want 10m", c.PollInterval)
 	}
+	if c.FetchBackoff != 30*time.Minute {
+		t.Errorf("FetchBackoff = %v, want 30m", c.FetchBackoff)
+	}
 }
 
 func TestLoad_RejectsBadDuration(t *testing.T) {