Convert completed events to transactions

Summary

Added atomic conversion of completed event costs into one dated cash expense transaction with a required date and expense section dialog, duplicate-conversion protection, and transaction status visibility.

Decisions

• Restrict conversion to essential, flexible, and daily expense sections; income is invalid for incurred event costs.
• Reject event totals above the transaction amount limit instead of truncating or splitting them.
• Store the created transaction ID on the event and preserve the event as an editable snapshot after conversion.

Learnings

• Completed event totals can exceed the transaction NUMERIC(14,2) maximum because event items aggregate independently.