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

features/seo_output_backend.qa.md

@@ -1,5 +1,7 @@
 # QA procedure: SEO output backend (slice 2 of 4)
 
+> Superseded in part by roles_permissions_enforcement: the owner and admin 403 clauses now answer 403 "missing_permission" with the header X-Missing-Permission, decided by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.
+
 Spec: `features/seo_output_backend.feature`. Go tests only, in `test/` (package `test`), run with `task test`. No live server, no real SMTP, no real SQLite file outside the temp-dir repository-contract harness. Call `validator.InitValidator()` before any handler call. Error assertions compare the plain-text body to the exact token or constant named in the spec. Success bodies are JSON. Assert exact key sets by decoding into `map[string]any` and comparing sorted key lists.
 
 ## Divergences from docs/design