feat: storage, language rules and migration

docs/api/articles.md

@@ -87,10 +87,14 @@ Breaking change: the default used to be HTML. Clients that need HTML must send `
   "noindex": false,
   "seo_dismissed": [],
   "created_at": "2026-03-12T09:14:00Z",
-  "updated_at": "2026-09-10T07:00:00Z"
+  "updated_at": "2026-09-10T07:00:00Z",
+  "lang": "fr-CA",
+  "story_id": 5
 }
 ```
 
+`lang` is the full BCP 47 language tag of the article (`""` while the site default language is not confirmed) and `story_id` is the story the article belongs to. Both are present on every article object (list items, reads, create and update responses, public reads) as additive keys, and every other key is unchanged. They are read-only: `lang` and `story_id` sent in a `POST` or `PATCH` body are ignored. Supported tags: `en`, `en-GB`, `en-US`, `fr`, `fr-CA`, `pt`, `pt-BR`, `es`, `de`, `ar`. Existing articles each get their own story and `lang` `""` until the site default is confirmed.
+
 `created_by` is the id of the user who created the article (`0` when it is attributed to nobody). It is present on every article object (list, read, create and update responses) only when the caller is signed in with a verified session or authenticates with an API key; the key acts as its creator. Anonymous callers, and unverified or stale sessions, get the same object without the field. The SPA reads it to decide which rows a role limited to `own` can edit, delete or publish.
 
 Every single-article read (`GET /v1/article/{id}`, `POST`, `PATCH`, `publish`, `unpublish`) carries the twelve SEO fields. Each item of `GET /v1/article/` carries `slug` and `excerpt` only, and none of the other ten.
@@ -116,7 +120,7 @@ Limits are counted in characters (runes), not bytes. Absent fields are untouched
 
 ### The slug
 
-A slug is unique per site (`site_id`, `slug`), never globally. At creation it is generated from the title: lowercase, accents folded, every run of other characters becomes one dash, dashes trimmed, cut to 100 characters without a trailing dash, and `article` when no letter or digit is left. A slug already used on the site gets `-2`, `-3`... silently, the base being truncated so the result fits 100 characters. The same rule backfills existing articles on the first start after the upgrade, in id order within each site.
+A slug is unique per site and language (`site_id`, `lang`, `slug`), never globally; unassigned articles (`lang` `""`) share the empty language, so they keep the per-site rule. At creation it is generated from the title: lowercase, accents folded, every run of other characters becomes one dash, dashes trimmed, cut to 100 characters without a trailing dash, and `article` when no letter or digit is left. A slug already used on the site gets `-2`, `-3`... silently, the base being truncated so the result fits 100 characters. The same rule backfills existing articles on the first start after the upgrade, in id order within each site.
 
 Editing a slug that another article of the target site already uses (the new site when `site_id` is sent in the same request; moving an article also re-checks its current slug there) answers `409` with the plain-text body `slug_taken`, and the response header `X-Slug-Suggestion` holds the first free `<slug>-N` from 2 on that site, fitting 100 characters. `X-Slug-Suggestion` is set on this `409` only and on no other response. A request with `slug: ""` that regenerates onto another article's slug is refused the same way and never suffixed silently. Re-sending an article's own slug is not a conflict. A refused request changes nothing.