feat(usecase): surface TranscriptSource on ProcessResult

The engine already distinguishes SourceNone from SourceRateLimited internally
but collapsed both into Skipped. Expose the source string so the runner can
persist the right transcript_status and apply rate-limit backoff, without the
engine taking on any store/retry concern (dependencies still point inward).

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

internal/usecase/engine.go

@@ -45,6 +45,11 @@ type ProcessResult struct {
 	Video   domain.Video
 	Skipped bool
 	Reason  string // set when Skipped (e.g. "no transcript")
+	// TranscriptSource is how the transcript resolved (or that there was none):
+	// the domain.TranscriptSource value as a string. The runner reads it to tell a
+	// permanent absence (SourceNone) from a transient 429 (SourceRateLimited) and
+	// persist the right transcript_status. Empty when a fetch error short-circuits.
+	TranscriptSource string
 	Summary          *domain.Summary // nil when Skipped
 }
 
@@ -59,7 +64,9 @@ func (e *Engine) ProcessNewVideo(ctx context.Context, v domain.Video) (ProcessRe
 	if !t.HasText() {
 		// No usable transcript: record the skip, produce no summary, deliver nothing
 		// (captions-first, ADR-007; the watcher uses this to avoid reprocessing).
-		return ProcessResult{Video: v, Skipped: true, Reason: "no transcript"}, nil
+		// Surface the source so the runner separates SourceNone (permanent) from
+		// SourceRateLimited (retry after a backoff window).
+		return ProcessResult{Video: v, Skipped: true, Reason: "no transcript", TranscriptSource: string(t.Source)}, nil
 	}
 
 	sum, err := e.AI.Summarize(ctx, v, t)
@@ -76,7 +83,7 @@ func (e *Engine) ProcessNewVideo(ctx context.Context, v domain.Video) (ProcessRe
 		}
 	}
 
-	return ProcessResult{Video: v, Summary: &sum}, errors.Join(errs...)
+	return ProcessResult{Video: v, Summary: &sum, TranscriptSource: string(t.Source)}, errors.Join(errs...)
 }
 
 // ProcessNewVideos walks a user's subscriptions and processes each newly seen