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  superseded/extended banners pointing at ADR-020 (kept as design records).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

@@ -179,3 +179,4 @@ distinguishable.
 | **"Summarize now" foreground path** | Unified quiet nudge button on actionable non-summarized cards. Five explicit card states — (1) summarized: chip + no button; (2) no captions (`transcript_status = 'none'`): "No transcript available", no button; (3) queued: "Queued" chip, no button; (4) rate-limited: "Fetching soon…" + "Summarize now" → `POST /v/{id}/retry-now` (clears `rate_limited_at`, triggers engine); (5) pending: "Not summarized" + "Summarize now" → `POST /v/{id}/summarize` (queues + triggers engine). One verb, one style (`.btn-quiet`); backend difference invisible to user. Both handlers call `ProcessVideo` through `globalFetchGate`. Rate gate respected, not bypassed — this is onboarding prioritisation. | Fast onboarding value; honest dead-end for no-captions videos (no button that fails). | `internal/web/handlers.go` (`handleRetryNow`, `handleRequestSummarize`); `internal/web/views.templ` (`VideoCard`) |
 | **Pipeline stats bar** | A one-line status bar above the video list: `N summarized · M fetching soon · K no captions`. Computed from the unfiltered row set; hidden when all videos are summarized. Gives the user a clear read on pipeline state without any interaction. | Replaces the "why is nothing happening?" confusion when most videos are pending or rate-limited. | `internal/web/view.go` (`PipelineStats`, `pipelineStats`) |
 | **Unavailable channels (account page)** | The `/account` page shows a "Unavailable channels" section when any channels returned HTTP 404 on the last discovery pass. Lists channel name, an "unavailable" badge, and the first-seen date. Data sourced from the `channel_errors` table (migration 013). | Surfaces silent failures so users know why some subscribed channels produce no new videos. | migration 013; `internal/web/account.go`; `internal/adapters/youtube/youtube.go` (`domain.ErrChannelUnavailable`) |
+| **Recency window + sparse-state honesty (ADR-020)** | Supersedes the copy/sort in the rows above. Auto-summarize is bounded to videos published within `TAPIR_AUTO_SUMMARIZE_WINDOW` (~7d); older un-summarized videos collapse behind a single "Show N older videos — summarize on demand" disclosure, and caption-less videos collapse to a one-line count (not N cards). List order is now `summarized-first, published_at DESC NULLS LAST`. Copy reframed for honest scarcity: pipeline bar reads "N ready · M in queue · K no captions" (no "fetching soon"); a gradual-fill note explains the rate limit; the nudge verb is "Summarize" (not "Summarize now"); the queued card says "summarizing shortly"; the empty-connected state drops the impossible `tapir run` instruction. Detail leads with Takeaways. Filters slimmed (no date pickers; hidden when empty); watched/skipped segmented; back link on detail; empty terms checkbox removed. | Make the sparse reality legible and honest instead of implying abundance/imminence; bound auto load so the back-catalogue doesn't re-drive the caption gate. Never fetch harder — scarcity is surfaced, not engineered around. | ADR-020; `2384c47`, `3df0459`, `40b703e`, `a1a5217`, `4a0a56e`, `9bf1c31`, `980638d`, `12fb031`, `f775441`, `51aa5d9` |