feat(API): media field type

docs/design/states.md

@@ -104,3 +104,6 @@ Keys: a type key is 2 to 40 lowercase letters, digits and hyphens (chosen at cre
 Schema changes with entries: Save opens the review ("Add 2 fields, change 1, remove 1"); Apply sends the PUT; schema_incompatible shows "Nothing was changed" and names the kind of change that likely caused it, never an entry. A type with entries cannot be deleted ("You can't delete Events yet").
 
 Entry rules, 422 except unique (409): required, type, min, max, options, unique, unknown_field. Messages are per type and drawn in the field-type gallery (group "Field types").
+
+## Media field (pass 5)
+Copy and states are in `media-field-changes.md` (sections 2 to 6). Entry rules for a media field: required 422 "Pick an image for Thumbnail."; type 422 "Thumbnail must be an image from the library."; min, max, options and unique do not apply. A required media field added to a type with entries is refused client side in the review ("Can't be applied") and by the server as 409 schema_incompatible. Image deleted while entries use it: the control shows "This image is no longer in the library"; saving is refused only when the field is required. HTTP statuses for the picker and the field use the table above (400, 401, 403, 404, 409, 413, 429, 500, network).