feat(API): media field type

docs/design/README.md

@@ -21,7 +21,7 @@ Back-office board groups: Sites, Sites: form, Sites: delete, Authors, Authors: f
 - Role-restricted variants drawn: read-only Authors list and detail, hidden Sites entry, Sites 403, own-author edit.
 
 ## Pass 4: custom content types
-Board `RenewCMS Screens Schema.dc.html` (frames: `SchemaFrame`, mounted in `AppFrame`): 9 groups, 65 states (56 available now, 9 later). 1 Schema list 6, 2 Schema builder 16, 3 Review and delete 7, 4 Field types 6, 5 Content and entry list 9, 6 Entry editor 13, 7 Relations 3, 8 Permissions 2, 9 Export and import 3. Plus tables: scope, routes, new components with their Nuxt UI mapping, errors to copy, needs backend (4 rows), decisions to confirm, accessibility. Specs: screens/schema.md, content.md, entry.md, schema-later.md.
+Board `RenewCMS Screens Schema.dc.html` (frames: `SchemaFrame`, mounted in `AppFrame`): 11 groups, 97 states. 1 Schema list 7, 2 Schema builder 19, 3 Review and delete 9, 4 Field types 7, 5 Content and entry list 12, 6 Entry editor 23, 7 Relations 3, 8 Permissions 2, 9 Export and import 3, 10 Media picker for a field 8, 11 Media library usage 4. The Media field type (pass 5) is specified in `media-field-changes.md`; the exact copy and measurements are there, and the boards are the visual reference. Plus tables: scope, routes, new components with their Nuxt UI mapping, errors to copy, needs backend (4 rows), decisions to confirm, accessibility. Specs: screens/schema.md, content.md, entry.md, schema-later.md, media.md, media-picker.md, and media-field-changes.md for the Media field type.
 - AppFrame gains a Schema item (owner and admin) and a Content group (sidebar and mobile drawer). Nothing else in AppFrame changed.
 - Backend gaps still open: built-in types and fields, display names for created_by and updated_by, a type icon, per-change entry counts.
 - Not done: Back-office and Prototype boards were not compared visually after the sidebar change (see Checks below).