docs: reconcile requirements + architecture with ADR-020

Bring the living docs current with the recency-bounded auto-summarize + sparse
honesty + feed IA bundle (ADR-020):

- requirements (BDD): summarize_mode.feature — auto now summarizes RECENT new
  videos; added a scenario for older videos (listed, on-demand), recency note.
- architecture.md: summarization-mode + new list-surface paragraph; scheduler
  diagram + two-path table + three-phase pass now show the recency pre-filter;
  dropped stale "Summarize now".
- data-model.md: auto_summarize is recent-only, older on-demand.
- README.md: one-line recency note on the serve scheduler.
- ui-spec.md: appended the as-built ADR-020 row (supersedes earlier copy/sort).
- specs/{video-card-states,newest-first-ordering,scheduled-discovery}.md:
  superseded/extended banners pointing at ADR-020 (kept as design records).

Docs-only; task check green.

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

README.md

@@ -75,8 +75,10 @@ password or Google); a new Dex subject is routed to `/register` to create a Tapi
 Stage 1 is multi-user: each user connects their own YouTube account from the browser and
 manages their own summaries under DB-enforced RLS isolation. When `TAPIR_DISCOVERY_INTERVAL`
 is set (e.g. `2h`), the serve process runs a scheduled discovery pass for every registered
-user automatically — no CronJob required. See `docs/homelab-integration.md` for the full
-config reference.
+user automatically — no CronJob required. In auto mode only videos published within
+`TAPIR_AUTO_SUMMARIZE_WINDOW` (default ~7d, ADR-020) are summarised automatically; older videos
+are listed and summarised on demand, so a large back-catalogue doesn't keep re-driving the
+caption rate gate. See `docs/homelab-integration.md` for the full config reference.
 
 ### Headless on koala