feat(calendar): single Default personal with guards and backfill

Ensure title Default; reject second personal and delete/type-change of
the sole personal; migrate extras to commercial or soft-delete.

Refs EventHub/EventHubBack#64

src/migrations/20260730200000_single_default_personal.erl

@@ -0,0 +1,11 @@
+%% @doc Backfill: ≤1 personal per owner, title Default; extras → commercial | delete.
+-module('20260730200000_single_default_personal').
+
+-export([up/0, down/0]).
+
+up() ->
+  logic_calendar:backfill_single_personal(),
+  ok.
+
+down() ->
+  ok.