feat: more role backend and admin/writer roles additions and adjustments

features/seo_data_backend.qa.md

@@ -1,5 +1,7 @@
 # QA procedure: SEO data backend (slice 1 of 4)
 
+> Superseded in part by roles_permissions_enforcement: the owner and admin 403 clauses now answer 403 "missing_permission" with the header X-Missing-Permission, decided by the caller's role permissions. An Admin can now invite and write sites, and a role without the needed permission is refused on routes that were open to every verified user.
+
 Spec: `features/seo_data_backend.feature`. Go tests only, in `test/` (package `test`), run with `task test`. No live server, no real SMTP, no real SQLite file outside the temp-dir repository-contract harness. Call `validator.InitValidator()` before any handler call. Error assertions compare the plain-text body to the exact token or constant named in the spec. The 409 `slug_taken` assertion checks the plain-text body `slug_taken` and the `X-Slug-Suggestion` header.
 
 ## Divergences from docs/design
@@ -7,7 +9,7 @@ Spec: `features/seo_data_backend.feature`. Go tests only, in `test/` (package `t
 `docs/design` is read-only; the SPA slices (3 and 4) align to the code, not to these design words.
 
 - **Site image and robots names:** the API uses `default_social_image_id` and `default_robots` (existing site contract). The design writes `social_image_id` for the site and, in places, `robots`. The SPA uses the code names.
-- **Non-owner site writes:** the API answers 403 with the existing text "Only the owner can do this." (`apperr.OwnerRequiredErrorMessage`). The design mentions a 403 `owner_only` token, which does not exist.
+- **Non-owner site writes:** the API answers 403 with the existing text "Only the owner can do this." (`apperr.MissingPermissionErrorMessage`). The design mentions a 403 `owner_only` token, which does not exist.
 - **Empty-slug fallback:** one rule everywhere. A title with no letter or digit becomes `article`, then `article-2`, `article-3` on conflict, in the API and in the migration. `docs/design/seo-slices.md` (decision 5) says `article-<id>` for the migration; that wording is not followed.
 - **Slug conflict transport:** the conflict follows the authors slug conflict (plain text `slug_taken`, 409); the suggestion travels in the `X-Slug-Suggestion` response header, not in a JSON body. The `slug.suggestion` of the slice 2 check endpoint is separate.
 - **Empty slug:** `slug: ""` regenerates the slug from the title (the SPA "Reset to match the title" sends `""`); 409 only if the regenerated slug belongs to another article on the site.