refac: authenticated user can upload an image to an existing article and receives its public location; an unknown article returns 404 and a disallowed type returns 400

features/upload_article_image.feature

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+Feature: Uploading an image to an article
+
+  As an authenticated RenewCMS user managing the CMS
+  I want to attach an image file to an existing article and get back the
+  image's public location
+  So that an unknown article answers 404 the same way every other article
+    write does, without ever creating an orphaned image, and a disallowed
+    content type is rejected with 400 before it becomes part of the
+    article, without a maintainer needing to trust the client to send a
+    sane file
+
+  Background:
+    Given internal/infrastructure/useCases.CreateImageUseCase.CreateImage
+      already checks the uploaded content type against the allowed list
+      (image/png, image/jpeg, image/webp, image/svg+xml) and the size
+      against the 10485760-byte maximum before calling image.Repository.Create,
+      returning image.ErrImageTypeNotAllowed or image.ErrImageTooLarge and
+      calling the repository not at all when either check fails
+    And internal/infrastructure/persistence's ArticleRepository.AddImage
+      already returns article.ErrArticleNotFound when the article id doesn't
+      match any row, and image.ErrImageNotFound when the article exists but
+      the image id doesn't, both asserted with errors.Is
+    And before this slice, api/handlers/image.Handler.PostImage existed and
+      was wired to CreateImageUseCase and UpdateArticleUseCase.AddImage, but
+      no route called it, and every error it got back from either use case
+      fell through to a blanket 500
+    And this slice mounts POST /v1/article/{id}/image in api/router.go, in
+      the same jwtauth Verifier + Authenticator group as the other article
+      write routes, and changes the upload flow so that the article's
+      existence is confirmed before the image is created, so that an
+      unknown article id never leaves an orphaned image behind
+    And this slice changes PostImage's error handling so that errors.Is(err,
+      image.ErrImageTypeNotAllowed) or errors.Is(err, image.ErrImageTooLarge)
+      answers 400 with the sentinel's own message, errors.Is(err,
+      article.ErrArticleNotFound) answers 404 with body
+      apperr.ArticleNotFoundErrorMessage, and any other error still falls
+      back to a generic 500
+    And PostImage reads the uploaded file from the multipart form field
+      "file"
+    And on success PostImage answers 201 with a JSON body {"location": ...}
+      holding the created image's public path, e.g.
+      "/static/uploadedImages/<stored-filename>"
+
+  Scenario: Uploading an allowed image type to an existing article succeeds
+    Given a valid jwt cookie for a user with id 1
+    And the article repository contains an article with id 1 titled
+      "Backyard Renovation Update"
+    When the client sends POST /v1/article/1/image with that cookie and a
+      multipart form field "file" holding a file named "before.png" with
+      content type "image/png"
+    Then the response status is 201
+    And the response body is JSON with a "location" field whose value is
+      "/static/uploadedImages/before.png"
+    And the image repository contains a new image whose Path is
+      "/static/uploadedImages/before.png"
+    And the article repository's article with id 1 has that new image's id
+      in its Images slice
+
+  Scenario: Uploading to an unknown article answers 404 and creates no image
+    Given a valid jwt cookie for a user with id 1
+    And the article repository has no article with id 9999
+    When the client sends POST /v1/article/9999/image with that cookie and a
+      multipart form field "file" holding a file named "before.png" with
+      content type "image/png"
+    Then the response status is 404
+    And the response body is the text "Article not found"
+    And the image repository's Create is never called
+    And no image exists in the image repository
+
+  Scenario: Uploading a disallowed content type answers 400 before the article is touched
+    Given a valid jwt cookie for a user with id 1
+    And the article repository contains an article with id 1 titled
+      "Backyard Renovation Update"
+    When the client sends POST /v1/article/1/image with that cookie and a
+      multipart form field "file" holding a file named "notes.pdf" with
+      content type "application/pdf"
+    Then the response status is 400
+    And the response body is the text
+      "the file must be a PNG, JPEG, WEBP, or SVG image"
+    And the image repository's Create is never called
+    And the article repository's article with id 1 still has an empty
+      Images slice
+
+  Scenario: Uploading an image still requires authentication
+    Given no jwt cookie is sent
+    And the article repository contains an article with id 1 titled
+      "Backyard Renovation Update"
+    When the client sends POST /v1/article/1/image without a cookie and a
+      multipart form field "file" holding a file named "before.png" with
+      content type "image/png"
+    Then the response status is 401
+    And the image repository's Create is never called
+    And the article repository's article with id 1 still has an empty
+      Images slice