chore: grounwork for SEO features

docs/design/routes.md

@@ -38,3 +38,6 @@
 
 ## Sites and authors in the URL
 `/articles?site=journal` filters by site key; it combines with `page`, `size`, `tab` and `q`, and resets `page` to 1 when it changes. An unknown key is dropped from the URL with no error. `/authors?q=camille&page=2&size=25` follows the same pagination rules as Articles. Deep links: `/authors?edit={slug}` and `/settings/sites?edit={key}` open the slide-over for owner and admin (authors) and the owner (sites); an unknown value shows the info toast "That author no longer exists" or "That site no longer exists". For other roles `/authors?edit={slug}` opens the read-only detail, and `/settings/sites` and its `?edit=` link show the 403 page "You don't have access to Sites".
+
+## SEO
+No new page routes. The panel and dialog are overlays on `/article/:id/edit`: `?panel=seo` (optionally `&field=seo_title|seo_description|slug|cover|authors|body` for the suggestion jump) and `?dialog=seo-api`. `/articles?seo=suggestions` is the Needs attention filter. API: GET /v1/article/{id}/seo, GET /v1/site/{key}/sitemap?page=&size=, PATCH /v1/article/{id} (new fields), PATCH /v1/site/{id} {site_name, title_template, default_description, social_image_id, default_robots}, GET /v1/article/?seo=ready|suggestions|hidden. 401 without a session; 403 owner_only on site SEO fields; 409 slug_taken {suggestion} on PATCH slug; 400 on invalid canonical_url or title_template.