feat: media picker and image removal

features/remove_legacy_article_image_alias.feature

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+Feature: The legacy POST /v1/article/{id}/image alias is removed
+
+  As the maintainer of RenewCMS
+  I want the editor to upload through /v1/media only and the article alias deleted
+  So that there is one way to add an image and no dead route to keep
+
+  Background:
+    Given this is a Go API change in a slice that is otherwise SPA only, made at the user's request
+    And features/upload_article_image.feature and its .qa.md are deleted, with their tests
+    And api/router.go no longer mounts r.Post("/{id}/image", ...) in NewArticleRouter, and the
+      imageHandler variable and the api/handlers/image import are removed from it
+    And the package api/handlers/image (PostLegacyArticleImage, its NewHandler and its helpers) is deleted
+    And CreateImageUseCase and GetArticleUseCase stay: both are used by the media handler and
+      the article handler, and api/pkg/upload stays because the media handler uses it
+    And the rest of the protected article group is unchanged, still behind jwtauth.Authenticator,
+      RequireCurrentToken and RequireVerifiedUser
+    And docs/architecture.md no longer lists api/handlers/image
+    And POST /v1/media/ is the only upload route, and DELETE /v1/article/{id}/image/{imageId}
+      stays unmounted, as before
+
+  Scenario Outline: The removed routes are not found
+    Given a valid jwt cookie for verified user 1 and an article with id 1
+    When the client sends <method> <path> with a valid PNG as multipart field "file"
+    Then the response status is 404
+    And the image repository's Create is never called
+    And no image exists afterwards
+
+    Examples:
+      | method | path                    |
+      | POST   | /v1/article/1/image     |
+      | DELETE | /v1/article/1/image/10  |
+
+  Scenario: Uploading still works through the media route
+    Given a valid jwt cookie for verified user 1
+    When the client sends POST /v1/media/ with a real PNG named "before.png"
+    Then the response status is 201 and the body is the library item with a "url" and an "id"
+    And GET /v1/media/ lists it
+
+  Scenario: The rest of the article router is unchanged
+    Given a valid jwt cookie for verified user 1 and an article with id 1
+    When the client sends PATCH /v1/article/1/publish
+    Then the response is as before this slice