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  superseded/extended banners pointing at ADR-020 (kept as design records).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

@@ -147,9 +147,10 @@ mechanism.
 
 - **USER** — one row per registered user (Stage 1, ADR-012; no longer single-row). The Tapir-side
   profile; the Dex identity is held separately in `USER_IDENTITY`, not on this row. `auto_summarize`
-  (migration 006) is the per-user mode flag: `TRUE` = auto-summarize every new video. Default is
-  **true** for new users (migration 011, ADR-018); existing rows were back-filled via migration 012
-  with RLS bypass.
+  (migration 006) is the per-user mode flag: `TRUE` = auto-summarize new videos **published within
+  the recency window** (`TAPIR_AUTO_SUMMARIZE_WINDOW`, default ~7d, ADR-020); older videos are
+  listed but summarised on demand. Default is **true** for new users (migration 011, ADR-018);
+  existing rows were back-filled via migration 012 with RLS bypass.
 - **USER_IDENTITY** (migration 004) — the `dex_subject → user_id` map. `dex_subject` is the PK,
   `user_id` a `UNIQUE` FK to `users` with `ON DELETE CASCADE`. This is the bridge resolved at login
   *before* a `user_id` is known, so it is **deliberately not RLS-enabled** (it holds no user data;