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/postgres/2026021100.sql

@@ -0,0 +1,5 @@
+-- Add alias support: parent_id links an alias identity to a parent account
+ALTER TABLE ident_switch
+	ADD parent_id integer DEFAULT NULL;
+
+CREATE INDEX IX_ident_switch_parent_id ON ident_switch(parent_id);