docs(bdd): scenario for transcript reuse — re-analysis never re-fetches

Capture the ADR-021 promise as a mapped BDD scenario: re-analyzing a
stored video reads the stored transcript and does not fetch captions.
Since paste-a-URL and the onboarding burst summarize through the same
engine chokepoint (resolveTranscript, store-first), this one scenario
covers their reuse path too — there is exactly one gated caption entry
point (youtube.FetchTranscript → WaitFetchGate) and one engine caller in
front of it, so the dedup is structural, not per-feature.

Mapped to TestProcessNewVideo_SecondSummarizeDoesNotRefetch.

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

test/acceptance/scenario_coverage_test.go

@@ -67,6 +67,7 @@ var scenarioCoverage = map[string]string{
 	"A subscribed channel posts a video with no usable transcript": "TestVideoWithNoTranscriptIsSkipped",
 	"A channel I am not subscribed to posts a video":               "TestUnsubscribedChannelVideoIsNotProcessed",
 	"The same video is not summarized twice":                       "TestAlreadySummarizedVideoIsNotReprocessed",
+	"Re-analyzing a stored video does not re-fetch its transcript": "TestProcessNewVideo_SecondSummarizeDoesNotRefetch",
 }
 
 var (