chore: groundwork for media purge system

docs/design/screens/media.md

@@ -1,7 +1,7 @@
 # Media
 
 - **Route:** `/media` (verified users)
-- **API:** GET /v1/media/ · POST /v1/media/ · PATCH /v1/media/{id} · DELETE /v1/media/{id} · POST /v1/media/merge · GET /v1/media/{id}/usage
+- **API:** GET /v1/media/purge-preview · POST /v1/media/purge · GET /v1/media/ · POST /v1/media/ · PATCH /v1/media/{id} · DELETE /v1/media/{id} · POST /v1/media/merge · GET /v1/media/{id}/usage
 - **Visual reference:** `source/RenewCMS Screens Back-office.dc.html`, group "Media", drawn by `source/MediaFrame.dc.html` (open it in a browser; each state is drawn at desktop and mobile, with a light/dark toggle).
 
 The library of every uploaded image. It replaces "images belong to one article": an image belongs to the library and can be used in any number of articles. Deleting an article never deletes images. The sidebar gains **Media** (`images` icon) between Articles and Users, breadcrumb "Media".
@@ -12,7 +12,7 @@ New route for verified users, in the sidebar between Articles and Users with the
 
 Header: title "Media", subtitle "Every image you've uploaded, ready to use in any article.", primary "Upload" button (`upload` icon) on the right. On mobile the button is a full-width 48 px button pinned to the bottom (it turns into "Try again in 0:42" while rate limited, and is replaced by the bulk bar while items are selected).
 
-Toolbar: tabs **All**, **Unused** and **Duplicates** (count badge of duplicate groups, only while groups exist), then search ("Search by filename"), sort and the grid/list toggle. Sort options: Newest (default), Oldest, Name, Largest. Below md the toolbar stacks: search, tabs, then sort and toggle.
+Toolbar: tabs **All**, **Unused** and **Duplicates** (count badge of duplicate groups, only while groups exist), then search ("Search by filename"), sort and the grid/list toggle. Sort options: Newest (default), Oldest, Name, Largest. On the Duplicates tab, owners and admins also get a secondary "Purge duplicates" button (`merge` icon) after the sort menu (full width, 44 px, in the stacked toolbar on mobile), hidden while there are no groups; it opens the PurgeDuplicatesDialog described in [settings](settings.md#media-cleanup-second-card). Below md the toolbar stacks: search, tabs, then sort and toggle.
 
 Grid (default): 5 columns on desktop, 2 on mobile, 16 px gap (10 px on mobile). Each card is one button: 4:3 thumbnail (checkerboard behind transparent PNG and SVG), filename (medium, ellipsis), `size · dimensions` (SVG dimensions read "Vector"), then badges. The accessible name reads "north-wall.jpg, 3.9 MB, used in 2 articles" or "…, unused". A "Select all" row with the image count sits above the grid.
 
@@ -149,6 +149,7 @@ Grid items are buttons, one tab stop each. Enter opens the detail panel, Space t
 | Upload: rejected files (413, 400) | Type and size are checked in the browser first, so nothing is sent. A server 413 or 400 for a file that got through shows the same row with its own message. |
 | Upload: too many requests (429) | Rate limited. Retry-After is counted down in the button. It re-enables at zero. Already-added files are kept. |
 | Duplicates: groups | The Duplicates tab. Groups of identical images side by side, each with its uploader, upload date and "Used in N articles". The suggested copy to keep is the most used, then the oldest. |
+| Duplicates: purge all | Owner or admin, groups exist. "Purge duplicates" opens the purge dialog (preview, confirm, progress, results), the same one as Settings "Purge now". "Merge into this one" is unchanged. |
 | Duplicates: empty | No groups. The Duplicates tab loses its count badge. |
 | Merge: confirm | "Merge into this one" on floorboards.jpg. The dialog says articles using the other copies will switch to this image and the other copies will be deleted, with the article count. |
 | Merge: in progress | Request in flight. The dialog can't be dismissed. |