feat(runner): bound auto-summarize to a recency window

In automatic mode the scheduler now only summarizes videos published within
TAPIR_AUTO_SUMMARIZE_WINDOW (default ~7d). Older videos are still discovered
and listed — they keep the manual "Summarize" affordance — but are not
auto-processed, so a large back-catalogue (the maintainer's ~256-deep queue)
stops self-inflicting 429s against the per-IP caption gate each cycle (UX
review B1, recency design).

- runner.WithAutoWindow + Stats.SkippedTooOld; tooOld() treats a zero window
  as disabled and an undated video as never-aged-out (processed, not stranded).
- An explicit manual request bypasses the bound even in auto mode (requested
  videos are loaded in auto mode when a window is active).
- Wired through cmdRun, the scheduler's per-user runner, sumStats, and pass
  logging. config: TAPIR_AUTO_SUMMARIZE_WINDOW (default 168h), .env.example.
- Account copy (A7) updated to match: "Automatic summarizes new videos from
  about the last week; older videos stay browsable — summarize on demand."

The rate gate is untouched; the manual path still serialises through it. This
bounds auto LOAD, it does not fetch harder.

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

cmd/tapir/main.go

@@ -133,11 +133,13 @@ func cmdRun(ctx context.Context, log *slog.Logger) error {
 	// the same per-egress-IP limiter as the web click-path.
 	youtube.SetFetchRate(cfg.FetchRate)
 
-	r := runner.New(engine.Source, st, engine, cfg.UserID, log, runner.WithBackoff(cfg.FetchBackoff))
+	r := runner.New(engine.Source, st, engine, cfg.UserID, log,
+		runner.WithBackoff(cfg.FetchBackoff),
+		runner.WithAutoWindow(cfg.AutoSummarizeWindow))
 
 	log.Info("starting run", "user", cfg.UserID, "model", cfg.SummarizerModel,
 		"gateway", cfg.GatewayURL, "poll_interval", cfg.PollInterval, "fetch_backoff", cfg.FetchBackoff,
-		"fetch_rate", cfg.FetchRate)
+		"fetch_rate", cfg.FetchRate, "auto_window", cfg.AutoSummarizeWindow)
 	return r.Loop(ctx, cfg.PollInterval)
 }