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/landing_page.feature

@@ -0,0 +1,28 @@
+Feature: Public landing page
+  As a first-time visitor
+  I want a public welcome page before I log in
+  So that I understand what Tapir is and how to get started without hitting a login wall
+
+  Scenario: An unauthenticated visit to the root is sent to the welcome page
+    Given I am not logged in
+    When I open the root path "/"
+    Then I am redirected to "/welcome"
+
+  Scenario: The welcome page invites an unauthenticated visitor to start
+    Given I am not logged in
+    When I open "/welcome"
+    Then I see a "Get Started" call to action
+
+  Scenario: An authenticated user on the welcome page sees their way in and out
+    Given I am logged in
+    When I open "/welcome"
+    Then I see a link to my summaries
+    And I see a way to log out
+
+  Scenario: Logging out returns to the welcome page
+    Given I am logged in
+    When I log out
+    Then I am returned to "/welcome"
+
+  # /welcome is mounted outside the auth guard so it is reachable without a session;
+  # the root and all data routes stay behind it (commits around the WelcomePage work).