fix(event): store multiple recurrence cancel exceptions

Use bag table for recurrence_exception so canceling several WEEKLY occurrences keeps all exceptions (set key was only master_id).

Refs EventHub/EventHubFront#42

src/migrations/20260730120000_recurrence_exception_bag.erl

@@ -0,0 +1,45 @@
+%% @doc recurrence_exception: set → bag, чтобы хранить несколько cancel на один master.
+-module('20260730120000_recurrence_exception_bag').
+
+-export([up/0, down/0]).
+
+-include("records.hrl").
+
+up() ->
+  case lists:member(recurrence_exception, mnesia:system_info(tables)) of
+    false ->
+      case mnesia:create_table(recurrence_exception, [
+        {disc_copies, [node()]},
+        {type, bag},
+        {attributes, record_info(fields, recurrence_exception)}
+      ]) of
+        {atomic, ok} -> ok;
+        {aborted, {already_exists, recurrence_exception}} -> ok;
+        {aborted, Reason} -> error({create_table_failed, Reason})
+      end;
+    true ->
+      case mnesia:table_info(recurrence_exception, type) of
+        bag ->
+          ok;
+        _ ->
+          Rows = mnesia:dirty_match_object(#recurrence_exception{_ = '_'}),
+          case mnesia:delete_table(recurrence_exception) of
+            {atomic, ok} -> ok;
+            {aborted, Reason1} -> error({delete_table_failed, Reason1})
+          end,
+          case mnesia:create_table(recurrence_exception, [
+            {disc_copies, [node()]},
+            {type, bag},
+            {attributes, record_info(fields, recurrence_exception)}
+          ]) of
+            {atomic, ok} -> ok;
+            {aborted, Reason2} -> error({create_table_failed, Reason2})
+          end,
+          lists:foreach(fun(R) -> mnesia:dirty_write(R) end, Rows),
+          ok
+      end
+  end.
+
+down() ->
+  %% Откат в set потерял бы все кроме одного exception на master — не делаем.
+  ok.