fix: address second round of PR review for scope prefix

- Use dynaconf (get_settings()) instead of os.getenv for OIDC_RESOURCE_SERVER_ID
- Re-add Settings field, _field_map entry, and settings.toml default
- Add trailing-slash guard (.rstrip("/")) to prevent double-slash in scopes
- Add double-prefixing guard: skip scopes already carrying the prefix
- Add @pytest.mark.unit to test module
- Add test for already-prefixed scopes
- Document OIDC_RESOURCE_SERVER_ID in docs/configuration.md

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

settings.toml

@@ -37,6 +37,7 @@ oidc_issuer = "@none"
 jwks_uri = "@none"
 introspection_uri = "@none"
 userinfo_uri = "@none"
+oidc_resource_server_id = "@none"
 
 # --- Mode flags ---
 enable_multi_user_basic_auth = false