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>

.env.example

@@ -51,6 +51,12 @@ TAPIR_POLL_INTERVAL=
 # caption endpoint; after it expires the video is retried. 0 = always retry.
 # Go duration; default 1h.
 TAPIR_FETCH_BACKOFF=
+# Recency bound for AUTO summarization: in automatic mode only videos published
+# within this window of now are summarized; older ones are discovered + listed
+# but wait for a manual "Summarize" (so a back-catalogue doesn't self-inflict
+# 429s). An explicit request bypasses it. Go duration; default 168h (~7d).
+# 0 = no bound (summarize every unseen video).
+TAPIR_AUTO_SUMMARIZE_WINDOW=
 # Minimum interval between outbound caption fetches across the WHOLE process —
 # the shared per-egress-IP rate gate (ADR-014). Scheduler runners and the web
 # "Summarize" click-path serialise through it so they cannot collectively trip