Add OIDC role sync config (OIDC_ADMIN_GROUP_NAME, OIDC_GROUPS_CLAIM)

Mv.OidcRoleSyncConfig reads from config; runtime.exs overrides from ENV in prod.

config/config.exs

@@ -58,6 +58,11 @@ config :mv,
     max_rows: 1000
   ]
 
+# OIDC group → role sync (optional). Overridden in runtime.exs from ENV in production.
+config :mv, :oidc_role_sync,
+  admin_group_name: nil,
+  groups_claim: "groups"
+
 # Configures the endpoint
 config :mv, MvWeb.Endpoint,
   url: [host: "localhost"],