test(authz): bearer-authed admin cannot edit/cover NULL-uploader mangas (0.87.19)

0.87.10 followup from the rereview. The fix lacked a test for the
specific axis it changed (admin authority via cookie only). New test
mints a bearer for a promoted admin and asserts PATCH/PUT-cover/DELETE-
cover all 403 on NULL-uploader rows. Mutation-confirmed.

Also: `MultipartBuilder::finalize` now `pub` for bearer-multipart use;
`admin_csrf_guard` rustdoc updated to describe the post-0.87.10
cookie-precedence rule.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.87.18",
+  "version": "0.87.19",
   "private": true,
   "type": "module",
   "scripts": {