feat(domain): add SourceRateLimited transcript source

429 from the caption endpoint means the IP is rate-limited (retry later),
not that the video has no captions. Distinguishing it from SourceNone is the
prerequisite for the runner's backoff/retry logic.

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

internal/domain/domain.go

@@ -18,6 +18,12 @@ type TranscriptSource string
 const (
 	SourceCaptions TranscriptSource = "captions"
 	SourceNone     TranscriptSource = "none"
+	// SourceRateLimited records that the caption endpoint returned HTTP 429.
+	// Unlike SourceNone (a permanent absence), this is a transient "retry later":
+	// the IP is rate-limited, not the video caption-less. It carries no text
+	// (HasText is false), so the engine degrades the same as SourceNone, but the
+	// runner persists it distinctly to retry after a backoff window.
+	SourceRateLimited TranscriptSource = "rate_limited"
 )
 
 // User is the Tapir-side profile. At Stage 0 there is exactly one.