chore: groundwork for SEO fixes and tech&vis debt

docs/design/screens/integration.md

@@ -16,7 +16,7 @@ Sticky section index on the right (desktop), "Jump to" select (mobile). A refere
 - Note 1: "A key can't create, list or revoke API keys, log in, register, verify or reset a password, or call anything under `/v1/system`. Those routes answer 403 "This action needs a real session." An invalid, unknown, expired or revoked key answers 401, even on public routes, and so does a key whose user was deleted or unverified."
 - Note 2: "Each key can make 120 requests per minute. Beyond that the API answers 429 with a `Retry-After` header."
 - Groups: Articles (7), Media (6), Users and invites (6), Settings (1), Session only (12 routes, 3 sub-groups; every `/v1/system` route is in this group). Session-only banner: "These routes need a signed-in browser session. With an API key they answer 403 "This action needs a real session.""
-- Errors: "Every error is a plain-text body with a single sentence. The status code is in the HTTP status line, and the Content-Type is text/plain." Table 400, 401, 403, 404, 429, 500 with the real messages below. Sample: a status line plus the text body (`HTTP/1.1 403 Forbidden`, `Content-Type: text/plain`, blank line, `This action needs a real session.`).
+- Errors: "Every error is a plain-text body with a single sentence. The status code is in the HTTP status line, and the Content-Type is text/plain." Table 400, 401, 403, 404, 429, 500 with the real messages below. The 403 row has a note line under it: "Site SEO defaults are owner only. Article SEO fields need only the right to edit the article." (12/18, `--ui-text-muted`, in the cause column, 2 px under the cause text). It is the only row with a note; rows below it move down by the wrapped note height (two 18 px lines on desktop, three on mobile, plus 2 px). Sample: a status line plus the text body (`HTTP/1.1 403 Forbidden`, `Content-Type: text/plain`, blank line, `This action needs a real session.`).
 - Notes: image URL prefix, CORS (existing copy), and "Article routes accept `?format=markdown`, `?format=html` or `?format=json` to choose how `body` is returned. With `json`, `body` is the stored JSON structure. Articles have `body_format: markdown` by default."
 
 ## EndpointCard states
@@ -34,8 +34,8 @@ Desktop 1440×900, mobile 390×844, light and dark.
 Plain text, one sentence. Used in the table and in the per-endpoint error captions.
 - 400: "The request cannot be processed due to a mismatch in the format of the body." / "The expiry must be a date in the future." Image requests (`/media/photo.png?v=`) answer 400 with a JSON body (`status`, `error`, `message`, `detail`) in two cases: `{ "status": 400, "error": "unknown_variant", "message": "Unknown variant", "detail": "unknown" }` for an unknown or disabled variant, and `{ "status": 400, "error": "unsupported_parameter", "message": "Unsupported parameter", "detail": "width" }` for any other query parameter (`width`, `height`, `format`, `quality`). Both are rows of the "Responses" table in "Images and variants".
 - 401: "You must be logged in to access this resource."
-- 403: "This action needs a real session." (session-only routes called with a key) / "Only the owner can do this." / "You must verify your account to access this resource."
-- 404: "Article not found" / "Image not found" / "Invite not found" / "API key not found"
+- 403: "This action needs a real session." (session-only routes called with a key) / "Only the owner can do this." / "You must verify your account to access this resource." Note under the 403 row: "Site SEO defaults are owner only. Article SEO fields need only the right to edit the article." The inline parenthetical that said the same thing in the row's cause text was removed to avoid saying it twice.
+- 404: "Article not found" / "Image not found" / "Invite not found" / "API key not found" / `site_not_found` (GET /v1/site/{key}/sitemap only; body is the code, not a sentence)
 - 429: "Too many requests. Please try again later." A per-key limit also sends `Retry-After`.
 - 500: "Internal server error"
 - 409, only as examples under their endpoints: `POST /v1/user/invite` "This address already has an account."; `POST /v1/media/merge` "Some of the images to merge changed. Refresh and try again." There is no 409 or 422 row in the table.
@@ -112,5 +112,5 @@ New section "Images and variants" on the Integration page, after Media and befor
 ## Search and sharing (SEO round)
 New group "Search and sharing" after Articles, in the sidebar index and the mobile Jump to select. Two cards in the existing endpoint style (collapsed, method badge, auth badge "Public", example, 401 and 404 rows, copy buttons):
 - GET /v1/article/{id}/seo: the resolved SEO object. Fields: title, description, canonical_url, robots, og{type,title,description,url,site_name,image,image_alt,locale}, twitter{card,title,description,image,image_alt}, image{url,alt,source}, authors[{name,url,same_as}], published_at, modified_at, fallbacks, json_ld. Drafts answer 404 to anonymous callers.
-- GET /v1/site/{key}/sitemap?page=&size=: yes, shown per site by key. Items: { url, modified_at, noindex }. Total in X-Total-Count. Sizes 25, 50, 100, 500, default 100. 404 "Site not found".
+- GET /v1/site/{key}/sitemap?page=&size=: yes, shown per site by key. Items: { url, modified_at, noindex }. Total in X-Total-Count. Sizes 25, 50, 100, 500, default 100. Errors: 400 "The request cannot be processed due to a mismatch in the format of the body." (same text as the other routes; bad `page` or `size`), 401, 404 `site_not_found` (unknown site key; this is the body the API returns, shown as a code and not rewritten as prose).
 PATCH /v1/article/{id} lists the new optional fields (slug, excerpt, seo_title, seo_description, cover_image_id, cover_image_alt, social_title, social_description, social_image_id, canonical_url, noindex, seo_dismissed). The 403 row of the HTTP status table adds: site SEO defaults are owner only; article SEO fields need only the right to edit the article.