feat(API): media library

docs/architecture.md

@@ -26,8 +26,15 @@ Nothing under `internal/` imports `api/`.
 | Package | Role |
 |---|---|
 | `api/handlers/user` | Authenticated user endpoints: profile update (`UpdateMe`) and the users list with invitations (`CreateInvite`, `ListUsers`). |
+| `api/handlers/media` | The media library under `/v1/media` (list, upload, rename, delete, merge, usage). Images belong to the library, not to an article: deleting an article never deletes an image, and an image's "used in" is read from article bodies each time. |
+| `api/handlers/image` | `POST /v1/article/{id}/image`, a temporary alias that creates a library image and attaches nothing. `PostLegacyArticleImage` goes away when the editor moves to `/v1/media`. |
+| `api/pkg/upload` | Multipart `file` reading and the mapping of upload errors (415, 413, 400) shared by both upload routes. |
 | `api/pkg/publicuser` | `PublicUser`, the JSON shape of a user with no password, codes or token version. `publicuser.New` converts a domain `user.User`, so handlers never serialise the domain struct directly. |
 
+### Image metadata backfill
+
+At boot, `BackfillImageMetadata` fills `filename`, `size`, `content_type`, `width`, `height` and `sha256` for image rows that lack them, reading the files in `UPLOAD_DIR`. It is idempotent, and a row whose file is missing or unreadable is left as it is without stopping the boot. The old `article_id` column and the `article_images` table are kept and never written.
+
 ## Frontend ↔ backend
 
 ```