Avoid calling EntityManager::flush() inside CalendarBalanceListener (stability/perf)

[algirdasc]

Summary

CalendarBalanceListener calls flush() inside Doctrine entity listeners (postPersist, postUpdate, postRemove). This pattern can cause nested flush side-effects, make transactions harder to reason about, and can severely degrade performance during bulk operations (imports, future recurring generators).

Affected code

• backend/src/EventListener/CalendarBalanceListener.php

Current behavior

Every Expense persist/update/remove triggers a listener that updates Calendar.balance and immediately flushes.

Problems

• Potential nested flush / UnitOfWork edge cases.
• Poor performance for bulk inserts/updates (flush per row).
• Harder to test/debug because side effects happen implicitly.

Suggested approaches

Option A (recommended):

• Move balance recomputation/update to the service layer where Expense changes occur; flush once per request/batch.

Option B:

• Use Doctrine onFlush/preFlush and schedule the calendar update without triggering a nested flush.

Option C:

• Remove materialized Calendar.balance and compute on-demand (may be too invasive).

Acceptance criteria

• No flush() calls occur inside entity listeners.
• Balance remains correct after create/update/delete and after bulk import.