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/scheduler.go

@@ -44,7 +44,9 @@ func buildUserRunner(cfg config.Config, st *store.Store, secretStore ports.Secre
 	}
 	engine := usecase.NewEngine(src, summarizer.New(primary, nil), st)
 
-	return runner.New(src, st, engine, userID, log, runner.WithBackoff(cfg.FetchBackoff)), nil
+	return runner.New(src, st, engine, userID, log,
+		runner.WithBackoff(cfg.FetchBackoff),
+		runner.WithAutoWindow(cfg.AutoSummarizeWindow)), nil
 }
 
 // userLister enumerates every registered user. *store.Store satisfies it via
@@ -85,7 +87,8 @@ func runDiscoveryPass(
 	log.Info("scheduler: pass complete",
 		"candidates", total.Candidates, "summarized", total.Summarized,
 		"skipped_seen", total.SkippedSeen, "skipped_no_text", total.SkippedNoText,
-		"skipped_manual", total.SkippedManual, "skipped_rate_limited", total.SkippedRateLimited,
+		"skipped_manual", total.SkippedManual, "skipped_too_old", total.SkippedTooOld,
+		"skipped_rate_limited", total.SkippedRateLimited,
 		"channel_unavailable", total.ChannelUnavailable, "errors", total.Errors)
 	return total
 }
@@ -129,6 +132,7 @@ func sumStats(a, b runner.Stats) runner.Stats {
 		SkippedSeen:        a.SkippedSeen + b.SkippedSeen,
 		SkippedNoText:      a.SkippedNoText + b.SkippedNoText,
 		SkippedManual:      a.SkippedManual + b.SkippedManual,
+		SkippedTooOld:      a.SkippedTooOld + b.SkippedTooOld,
 		SkippedRateLimited: a.SkippedRateLimited + b.SkippedRateLimited,
 		ChannelUnavailable: a.ChannelUnavailable + b.ChannelUnavailable,
 		Errors:             a.Errors + b.Errors,