Return 404 for legacy repository archive URL

What does this MR do and why?

Contributes to #595699

Problem

npm's hosted-git-info library generates tarball URLs in the legacy format /{namespace}/{project}/repository/archive.tar.gz. This URL does not match any Rails route and falls through to the catch-all *unmatched_route handler, which redirects unauthenticated users to /users/sign_in (302 → 200 HTML). npm then tries to parse the HTML sign-in page as a tarball, failing with TAR_BAD_ARCHIVE.

Solution

Add a dedicated route for the legacy repository/archive URL that maps to a new Projects::LegacyArchiveController#not_found action, which always returns 404. This prevents the confusing redirect-to-sign-in behavior and gives automated tools a proper HTTP error code. The controller inherits from ::ApplicationController (not Projects::ApplicationController) to avoid the before_action :project chain that triggers find_routable! and the redirect.

References

• #595699
• npm upstream issues: https://github.com/npm/cli/issues/3229, https://github.com/npm/pacote/issues/476
• npm's hosted-git-info tarball URL template for GitLab

Screenshots or screen recordings

Before	After
curl -sI .../repository/archive.tar.gz → 302 redirect to /users/sign_in → 200 HTML	curl -sI .../repository/archive.tar.gz → 404 Not Found

How to set up and validate locally

1. Create or use any private project (e.g. root/my-project)
2. Before this MR:

   curl -sI "http://gdk.test:3000/root/my-project/repository/archive.tar.gz?ref=main"
   # HTTP/1.1 302 Found — redirects to /users/sign_in

3. Enable FF: Feature.enable(:legacy_archive_not_found)
4. After this MR:

   curl -sI "http://gdk.test:3000/root/my-project/repository/archive.tar.gz?ref=main"
   # HTTP/1.1 404 Not Found

5. Verify the correct /-/archive/ URL still works for public projects:

   curl -sI "http://gdk.test:3000/root/my-project/-/archive/main/my-project-main.tar.gz"
   # HTTP/1.1 200 OK (for public projects)

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