feat: storage, language rules and migration

docs/architecture.md

@@ -94,9 +94,9 @@ The use cases `ListAuthors`, `CreateAuthor`, `CreateMyAuthor`, `UpdateAuthor`, `
 
 `internal/domain/slug` holds the one slug rule (`Generate`, `WithSuffix`, `IsValid`: lowercase, accents folded, runs of other characters become one dash, 100 characters, a caller-chosen fallback). `internal/domain/article` uses it with the fallback `article` and holds the twelve SEO fields of `Article`, the `Update` fields for them (`Nullable` for the two image ids), `ValidateCanonicalURL`, the eleven dismiss codes and `NormalizeDismissed`, and the sentinels `ErrInvalidSlug`, `ErrSlugTaken` and `ErrInvalidCanonicalURL`. `*SlugTakenError` wraps `ErrSlugTaken` and carries the suggested free slug; the handler turns it into the `X-Slug-Suggestion` header of the 409. `author.GenerateSlug` delegates to the same package.
 
-Slugs are unique per site, enforced by the unique index `idx_articles_site_slug` on `(site_id, slug)`. The use cases decide first: `CreateArticle` generates the slug from the title and takes the first free `-N` on the default site (retrying when the repository reports a race), `UpdateArticle` re-checks the slug on the target site for an edited slug, an empty slug (regenerated from the title) and a move, and computes the suggestion as the first free `-N` from 2, fitting 100 characters. `ArticleRepository` locks the site row, checks the slug inside the transaction and re-explains a failed write as `ErrSlugTaken`. Deleting an article replaces its slug with the tombstone `#deleted-<id>` (it can never match a valid slug) before the soft delete, so the slug is free again while the unique index keeps covering soft-deleted rows.
+Slugs are unique per site and language, enforced by the unique index `idx_articles_site_slug` on `(site_id, lang, slug)` (the name is kept from the earlier `(site_id, slug)` index, which the migration replaces once, tracked by the `article_slug_index_per_language` marker). Each article belongs to a story (`stories` table) and a second unique index `idx_articles_story_lang` on `(story_id, lang)` allows one version per language per story; both indexes are created after the `article_stories` backfill marker, which gives every pre-existing article its own story. `site_languages` holds one default language per site, seeded once from `Site.Locale` as an unconfirmed suggestion (`site_language_suggestions` marker). `CreateArticle` owns the rule that a new article takes the confirmed default: it runs through `transaction.Transactor`, asks `language.Repository.LockStartingLanguage` (which locks the site row and returns the confirmed default tag, or `""` while it is unconfirmed), picks the free slug for that language and calls `Create` with the resulting `Lang`, all in one transaction. `ArticleRepository.Create` only stores what it receives and opens the new story in the same transaction (the story rolls back with the article, `content_revision` starts at 1). `SiteLanguageRepository.ConfirmDefault` (lock, read the default, guard, flag it confirmed, assign unassigned articles) holds the same site lock, so no article is left unassigned once the confirmation commits. The "default and confirmed" predicate lives in one repository helper shared by both. `article.Repository.SlugTaken` takes the language, matching the index. The use cases decide first: `CreateArticle` generates the slug from the title and takes the first free `-N` on the default site (retrying when the repository reports a race), `UpdateArticle` re-checks the slug on the target site for an edited slug, an empty slug (regenerated from the title) and a move, and computes the suggestion as the first free `-N` from 2, fitting 100 characters. `ArticleRepository` locks the site row, checks the slug inside the transaction and re-explains a failed write as `ErrSlugTaken`. Deleting an article replaces its slug with the tombstone `#deleted-<id>` (it can never match a valid slug) before the soft delete, so the slug is free again while the unique index keeps covering soft-deleted rows.
 
-`database.Migrate` adds the columns with `AutoMigrate`, then runs `BackfillSiteTitleTemplates` and `BackfillArticleSlugs` (only rows with an empty slug, in id order, per site, applying the same rule and the same suffixes, with the tombstone for soft-deleted rows) and only then creates the unique index, so it is idempotent and completes a partly backfilled table. The default `title_template` is written by the backfill and the seed, not by a column default, because the SQLite migrator rebuilds the table on every start when the default contains `%` and `|`.
+`database.Migrate` adds the columns with `AutoMigrate`, then runs `BackfillSiteTitleTemplates` and `BackfillArticleSlugs` (only rows with an empty slug or a duplicate within the same site and language, in id order, per site and language, applying the same rule and the same suffixes, with the tombstone for soft-deleted rows) and only then creates the unique index, so it is idempotent and completes a partly backfilled table. The default `title_template` is written by the backfill and the seed, not by a column default, because the SQLite migrator rebuilds the table on every start when the default contains `%` and `|`.
 
 Media usage has more sources than article bodies: `Article.CoverImageID` and `SocialImageID`, `Author.AvatarID` and `Site.DefaultSocialImageID`. `ListMedia`, the purge plan and `GetMediaUsage` count an article once however many places use the image. `DeleteMedia` clears the cover and social references (`article.Repository.ClearImage`, restored with `RestoreImage` when the delete fails), `MergeMedia` repoints them (`RepointImage`) and counts an article rewritten in its body and its cover once, and `GetMediaUsage` (now built with the site and author repositories) returns articles, authors and sites with `type` and `fields`.