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  superseded/extended banners pointing at ADR-020 (kept as design records).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>

docs/specs/newest-first-ordering.md

@@ -1,5 +1,11 @@
 # Spec — Newest-first batch ordering + honest "Try now" / prioritisation docs
 
+> **Extended by ADR-020 (2026-06-08).** This spec covers the *batch processing* order within a
+> pass. ADR-020 adds (a) a recency pre-filter — auto mode skips videos published before
+> `TAPIR_AUTO_SUMMARIZE_WINDOW`, listed but summarised on demand — and (b) the same
+> `published_at DESC NULLS LAST` ordering on the **list read** (`ListVideos`), which previously
+> sorted by `seen_at`. See `DECISIONS.md` ADR-020.
+
 **Repo:** tapir · **Size:** small · **Solo session.**
 
 **Why.** Product intent (maintainer, 2026-06-06): a new user should get summaries of their