mod_storage_internal,_sql: Expose archive capabilities feature set

This was planned to be added long ago but was forgotten.

plugins/mod_storage_internal.lua

@@ -47,6 +47,12 @@ end
 local archive = {};
 driver.archive = { __index = archive };
 
+archive.caps = {
+	total = true;
+	quota = archive_item_limit;
+	truncate = true;
+};
+
 function archive:append(username, key, value, when, with)
 	when = when or now();
 	if not st.is_stanza(value) then

plugins/mod_storage_sql.lua

@@ -232,6 +232,8 @@ end
 local archive_store = {}
 archive_store.caps = {
 	total = true;
+	quota = archive_item_limit;
+	truncate = true;
 };
 archive_store.__index = archive_store
 function archive_store:append(username, key, value, when, with)