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>

internal/usecase/engine.go

@@ -27,6 +27,13 @@ type Engine struct {
 	AI     ports.Summarizer
 	Sinks  []ports.Sink
 
+	// Transcripts, when set, is the shared transcript cache (ADR-021): the engine
+	// reads it before any caption fetch and writes resolved transcripts back, so
+	// re-analysis — the same user re-summarizing, or a second user with the same
+	// video — never re-touches YouTube (ADR-010/014). Optional: nil disables
+	// persistence (fetch every time), keeping the pure-core/scaffold wiring valid.
+	Transcripts ports.TranscriptStore
+
 	// processed dedups videos within this engine's lifetime so a video is not
 	// summarized twice when the watcher sees it again. Durable cross-restart
 	// dedup is the store's concern (a resolved TRANSCRIPT / existing SUMMARY,
@@ -57,9 +64,9 @@ type ProcessResult struct {
 // resolve transcript -> (summarize -> deliver) | skip.
 // See docs/use-cases/summarize_new_video.feature.
 func (e *Engine) ProcessNewVideo(ctx context.Context, v domain.Video) (ProcessResult, error) {
-	t, err := e.Source.FetchTranscript(ctx, v)
+	t, err := e.resolveTranscript(ctx, v)
 	if err != nil {
-		return ProcessResult{Video: v}, fmt.Errorf("fetch transcript: %w", err)
+		return ProcessResult{Video: v}, err
 	}
 	if !t.HasText() {
 		// No usable transcript: record the skip, produce no summary, deliver nothing
@@ -86,6 +93,39 @@ func (e *Engine) ProcessNewVideo(ctx context.Context, v domain.Video) (ProcessRe
 	return ProcessResult{Video: v, Summary: &sum, TranscriptSource: string(t.Source)}, errors.Join(errs...)
 }
 
+// resolveTranscript returns v's transcript, reading the shared store first
+// (ADR-021): a stored transcript — including a stored SourceNone (captions
+// permanently absent) — is returned without touching YouTube, so re-analysis
+// never re-fetches. On a store miss it fetches through the source (which gates
+// the caption call, ADR-014) and persists the terminal outcome so the next
+// analysis, for any user, reads from the store. A transient SourceRateLimited is
+// returned to the caller (the runner stamps a per-user backoff) but never stored,
+// so persistence can never mask a 429 as a permanent "no transcript". When no
+// TranscriptStore is wired the engine simply fetches every time.
+func (e *Engine) resolveTranscript(ctx context.Context, v domain.Video) (domain.Transcript, error) {
+	if e.Transcripts != nil {
+		stored, ok, err := e.Transcripts.GetTranscript(ctx, string(v.Provider), v.ProviderVideoID)
+		if err != nil {
+			return domain.Transcript{}, fmt.Errorf("get stored transcript: %w", err)
+		}
+		if ok {
+			return stored, nil
+		}
+	}
+
+	t, err := e.Source.FetchTranscript(ctx, v)
+	if err != nil {
+		return domain.Transcript{}, fmt.Errorf("fetch transcript: %w", err)
+	}
+
+	if e.Transcripts != nil && t.Source != domain.SourceRateLimited {
+		if err := e.Transcripts.SaveTranscript(ctx, string(v.Provider), v.ProviderVideoID, t); err != nil {
+			return domain.Transcript{}, fmt.Errorf("save transcript: %w", err)
+		}
+	}
+	return t, nil
+}
+
 // ProcessNewVideos walks a user's subscriptions and processes each newly seen
 // video. Only videos surfaced via the user's subscriptions are considered, so a
 // channel the user is not subscribed to is never processed. A video already