docs: consolidate design change logs and refresh stale docs

features/seo_data_backend.qa.md

@@ -12,7 +12,7 @@ Spec: `features/seo_data_backend.feature`. Go tests only, in `test/` (package `t
 
 - **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.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.
+- **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/decisions.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.
 - **Status codes:** `invalid_url` is 422 (authors); `invalid_slug`, `invalid_title_template` and `image_not_found` are 400.