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>

internal/web/views.templ

@@ -411,9 +411,10 @@ templ AccountPage(displayName, email string, conns []store.Connection, autoSumma
 			<section>
 				<h2>Summarization</h2>
 				<p class="muted">
-					Automatic summarizes every new video as it is discovered. Manual lets you
-					pick which videos to summarize — new videos appear in your list with a
-					Summarize button.
+					Automatic summarizes new videos from about the last week as they are
+					discovered. Older videos stay browsable — summarize them on demand.
+					Manual lets you pick which videos to summarize — every new video appears
+					in your list with a Summarize button.
 				</p>
 				@summarizeModeControl(autoSummarize)
 			</section>