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  superseded/extended banners pointing at ADR-020 (kept as design records).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/use-cases/summarize_mode.feature

@@ -6,12 +6,19 @@ Feature: Choose how new videos get summarized
   Background:
     Given I am a registered user with a connected video account
 
-  Scenario: Auto mode summarizes every new video
+  Scenario: Auto mode summarizes recent new videos automatically
     Given my summarization mode is "auto"
-    When a subscribed channel posts a new video with captions
+    When a subscribed channel posts a new video with captions within the recency window
     Then Tapir summarizes it without my asking
     And the summary appears in my list
 
+  Scenario: Auto mode lists older videos without summarizing them
+    Given my summarization mode is "auto"
+    When discovery finds a video published before the recency window
+    Then the video appears in my list with no summary
+    And it is not summarized automatically
+    And I can still summarize it on demand with "Summarize"
+
   Scenario: Manual mode is the default and leaves new videos unsummarized
     Given I have not changed my summarization mode
     Then my mode is "manual"
@@ -30,3 +37,8 @@ Feature: Choose how new videos get summarized
   # auto_summarize is a per-user setting and summarize_requested is a per-video queue
   # flag (migration 006). The web button sets the flag; `tapir run` processes both the
   # auto videos and the manually queued ones, then clears the flag.
+  #
+  # Recency bound (ADR-020): in auto mode the scheduler only summarizes videos published
+  # within TAPIR_AUTO_SUMMARIZE_WINDOW (default ~7d); older videos are discovered and
+  # listed but wait for an explicit "Summarize" — so a back-catalogue does not re-drive
+  # the per-IP caption gate (ADR-014) every cycle. A manual request bypasses the bound.