feat(usecase): read stored transcript before fetching (ADR-021)

The engine now resolves transcripts store-first: a stored transcript —
including a stored SourceNone — is summarized without touching YouTube,
so re-analysis never re-fetches. On a miss it fetches through the source
(caption call still gated, ADR-014) and persists the terminal outcome for
the next analysis by any user. A transient SourceRateLimited is surfaced
to the runner for per-user backoff but never cached, so persistence can
never mask a 429 as a permanent "no transcript".

The TranscriptStore is optional (nil → fetch every time), keeping the
pure-core and scaffold wiring valid. cmd/tapir wires the store as both
summary sink and transcript cache, so `tapir run` and the web summarize
path (incl. paste + onboarding) all share the dedup.

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

cmd/tapir/processor.go

@@ -63,7 +63,12 @@ func buildProcessor(cfg config.Config, st *store.Store) (*usecase.Engine, error)
 	}
 	sum := summarizer.New(primary, nil)
 
-	return usecase.NewEngine(src, sum, st), nil
+	// The store is both the summary sink and the shared transcript cache (ADR-021):
+	// the engine reads stored transcripts before any caption fetch and writes
+	// resolved ones back, so re-analysis never re-touches YouTube.
+	eng := usecase.NewEngine(src, sum, st)
+	eng.Transcripts = st
+	return eng, nil
 }
 
 // engineProcessor adapts the engine (which works in terms of a domain.Video) to