feat: SEO backend data (1/4)

docs/release-notes.md

@@ -15,6 +15,31 @@ Body, in this order. Omit a section only when it would be empty, except Upgradin
 
 Rules: bullets are full sentences ending with a period; user-facing wording, no commit prefixes; sentence-case headings; no horizontal rules, no metadata block, no "Pre-1.0" boilerplate (the prerelease flag says it); every release ships all 6 binaries from `task release`.
 
+## Next release (draft)
+
+Articles and sites now store the data a website needs for search and sharing, and every article gets an address that is unique on its site.
+
+### New
+
+**Article SEO fields**
+
+- An article has a slug, an excerpt, a search title and description, a cover image with its alt text, a social title, description and image, a canonical address, a noindex flag and a list of dismissed suggestions.
+- The slug is generated from the title, is unique per site and can be changed at any time; an empty slug regenerates it from the title.
+- A site has a site name, a title template and a default description.
+
+### API
+
+- `PATCH /v1/article/{id}` also takes the SEO fields. `cover_image_id` and `social_image_id` accept `null`. A slug already used on the target site answers `409` `slug_taken` with the header `X-Slug-Suggestion` holding the first free slug.
+- `GET /v1/article/{id}`, `POST /v1/article/`, `PATCH /v1/article/{id}`, `PATCH /v1/article/{id}/publish` and `PATCH /v1/article/{id}/unpublish` return the twelve SEO fields; the items of `GET /v1/article/` return `slug` and `excerpt`.
+- `POST /v1/site/` and `PATCH /v1/site/{id}` take `site_name`, `title_template` and `default_description`; a template without `%title%` is `400` `invalid_title_template`.
+- Changed: `GET /v1/media/{id}/usage` stays an array, and every item gains `type` and `fields` without losing `id`, `title` and `published`. Authors and sites that use the image are now listed, without a `published` key.
+- Changed: a cover image, a social image, an author avatar and a site default social image all count in `used_in` and in the `unused` filter. Deleting an image clears the covers and social images that use it, and merging or purging repoints them.
+
+### Upgrading
+
+- The first start after the upgrade adds the new columns, gives every existing article a slug generated from its title (in id order within each site, with `-2`, `-3` on conflicts) and creates the unique `(site_id, slug)` index. Nothing else changes, and a second start changes nothing.
+- A deleted article now frees its slug.
+
 ## Published releases
 
 Each body lives in `docs/releases/<tag>.md`. Publish with `tea releases create --tag <tag> --title "<tag> - <theme>" --note-file docs/releases/<tag>.md --prerelease --asset <file>` after `task release`, with one `--asset` per file: the six binaries, `bin/checksums.txt` and `bin/checksums.txt.sig`. `task release` produces all eight and fails before building when the signing key is missing or wrong (see [deployment.md](deployment.md#release-signing-key-maintainers-once)). Never publish a release without the two checksum files.