feat(web): collapse older + caption-less videos in the list

Stop the un-summarized back-catalogue from burying the readable summaries
(UX review B3/B4). One feed, with a noise-collapse — not sections:

- Summarized + recent un-summarized videos lead inline as cards.
- Un-summarized videos older than the recency window collapse into a single
  "Show N older videos — summarize on demand" disclosure (they will not
  auto-fill; they are manual-only). Window comes from App.RecencyWindow
  (= cfg.AutoSummarizeWindow); 0 disables the collapse (all inline).
- Caption-less videos collapse into one honest line ("N videos have no
  captions and can't be summarized") instead of N dead terminal cards.

bucketRows is a pure classifier (cutoff-driven; undated rows never age out);
App gains RecencyWindow + an injectable clock for the cutoff.

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

cmd/tapir/main.go

@@ -193,7 +193,7 @@ func cmdServe(ctx context.Context, log *slog.Logger) error {
 	// The file-backed SecretStore is shared by the connect flow (writes tokens)
 	// and account management (deletes them on disconnect / delete-account).
 	secretStore := secrets.NewFileStore(cfg.SecretsFile)
-	app := &web.App{Store: st, Identity: st, Auth: authn, Secrets: secretStore, Log: log}
+	app := &web.App{Store: st, Identity: st, Auth: authn, Secrets: secretStore, Log: log, RecencyWindow: cfg.AutoSummarizeWindow}
 
 	// User onboarding is handled by the IdP (Authentik invite flow), not Tapir —
 	// the Dex local-password provisioning path was removed (ADR-019). An