feat: more role backend and admin/writer roles additions and adjustments

features/image_variants_per_image_api.qa.md

@@ -1,5 +1,7 @@
 # QA procedure: image_variants_per_image_api
 
+> Superseded in part by roles_permissions_enforcement: the owner and admin 403 clauses now answer 403 "missing_permission" with the header X-Missing-Permission, decided by the caller's role permissions. An Admin can now invite and write sites, and a role without the needed permission is refused on routes that were open to every verified user.
+
 Go only, no SPA. Package `test` (black-box). New files: `test/image_variants_per_image_http_test.go`, `test/image_variants_generate_on_upload_test.go`, `test/media_variants_list_http_test.go`, `test/image_variants_per_image_audit_test.go`. Extend `test/image_variants_support_test.go`; fake type names must stay unique in the package. No live server, no real SQLite file, no real SMTP, no `time.Sleep`.
 
 ## Setup shared by every case