docs(use-cases): add registration, summarize-mode, landing scenarios

The .feature spec lagged shipped behaviour. Added three files, no
duplication of existing scenarios:
- registration.feature: new Dex subject -> registration gate (users +
  user_identities), returning subject straight through, account delete
  is tapir-side only and leaves other users intact, clean
  re-registration (ADR-012, ADR-013).
- summarize_mode.feature: auto summarizes every new video; manual
  (default) leaves them unsummarized until queued; queued video is
  processed and the flag cleared (migration 006).
- landing_page.feature: unauthenticated / -> /welcome, Get Started for
  guests, summary link + logout for authed users, logout -> /welcome.

Scoped to built features only — no Vimeo/Whisper/billing scenarios.

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

docs/use-cases/summarize_mode.feature

@@ -0,0 +1,32 @@
+Feature: Choose how new videos get summarized
+  As a user who wants control over compute and noise
+  I want to pick whether new videos are summarized automatically or on demand
+  So that I only spend summarization on the videos I actually care about
+
+  Background:
+    Given I am a registered user with a connected video account
+
+  Scenario: Auto mode summarizes every new video
+    Given my summarization mode is "auto"
+    When a subscribed channel posts a new video with captions
+    Then Tapir summarizes it without my asking
+    And the summary appears in my list
+
+  Scenario: Manual mode is the default and leaves new videos unsummarized
+    Given I have not changed my summarization mode
+    Then my mode is "manual"
+    When a subscribed channel posts a new video with captions
+    Then the video appears in my list with no summary
+    And nothing is summarized until I request it
+
+  Scenario: Requesting a summary in manual mode queues it for the next run
+    Given my summarization mode is "manual"
+    And a new video is in my list with no summary
+    When I click "Summarize" on that video
+    Then the video is marked as requested
+    And the next run summarizes it
+    And the request flag is cleared after it is processed
+
+  # 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.