Add parent_id column for alias identity support

Migration and initial schemas for linking alias identities to parent
accounts via a nullable parent_id foreign key.

SQL/mysql/2026021100.sql

@@ -0,0 +1,9 @@
+-- Add alias support: parent_id links an alias identity to a parent account
+ALTER TABLE `ident_switch`
+	ADD `parent_id`
+		int UNSIGNED
+		DEFAULT NULL
+		AFTER `iid`;
+
+ALTER TABLE `ident_switch`
+	ADD INDEX `IX_ident_switch_parent_id` (`parent_id`);