A month is a snapshot of the budget. MonthEntry holds the copied planned
amount plus applied and origin_name/origin_planned so the UI can mark
edited rows. source_entry_id links back to the budget but is nullable
with ON DELETE SET NULL, so deleting a budget row after snapshot leaves
the month intact. MonthDebtTarget is one row per month via CASCADE from
month.
Refs #3
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
