feat: SEO backend data (1/4)

docs/architecture.md

@@ -81,6 +81,16 @@ The use cases `ListSites`, `CreateSite`, `UpdateSite`, `SetDefaultSite` and `Del
 
 The use cases `ListAuthors`, `CreateAuthor`, `CreateMyAuthor`, `UpdateAuthor`, `DeleteAuthor` and `GetMyAuthor` live in `internal/infrastructure/useCases`; `CreateArticle` credits the creator's author, `UpdateArticle` validates and records `author_ids`, and the media, purge and activity use cases take the author repository as a required constructor argument (avatars count as media usage, are cleared on delete and repointed on merge and purge). `api/handlers/author` serves `/v1/author`: the list accepts a session or an API key, every write is session only, `POST /` and `DELETE` sit behind `RequireOwnerOrAdmin`, and the edit rights (owner or admin, or the caller's own linked author) are decided by `UpdateAuthor` from the caller the handler passes in. `GET /v1/auth/me` adds the linked author through `GetMyAuthor`.
 
+### Article SEO data
+
+`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.
+
+`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 `|`.
+
+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`.
+
 ### Image variants
 
 `internal/domain/image` holds the `Variant` entity and its rules (`VariantSettings.Normalized`, the settings `Fingerprint`, `VariantETag`, `ETagMatches`, `IsAnimatedGIF`), the sentinels (`ErrVariantNotFound`, `ErrVariantExists`, `ErrVariantInvalid`, `ErrVariantFormatUnavailable`, `ErrUnknownVariant`, `ErrUnsupportedParameter`, `ErrSourceTooLarge`) and three ports: `VariantRepository` (definitions), `VariantFileRepository` (the index of generated files: variant name, source stored name, cached stored name, content type, size and the settings fingerprint, needed because `Storage.Put` chooses the stored name itself) and `VariantRenderer` (decode a source, encode a variant, report the formats it can encode). `internal/infrastructure/imaging` is the renderer adapter, free of GORM and HTTP; the persistence adapters are `VariantRepository` and `VariantFileRepository` over `models.ImageVariant` and `models.ImageVariantFile`.