Posting is a child of MonthEntry with occurred_on, amount, optional
description and payee. Cascade delete so removing an entry wipes its
ledger. Ordered on load by occurred_on DESC for readable UIs.
MonthEntry.applied becomes a @property summing posting amounts. The
stored applied column is dropped in the same migration.
The migration walks existing month_entry rows: for every non-zero
applied value, it inserts one opening-balance posting on the month's
activated_at (or created_at) date with description "opening balance"
and amount equal to the existing applied. Empty applied values get
no opening posting. Closed months go through the same path; their
totals stay intact via that single seeded row.
Downgrade is symmetric: re-adds the column and populates from
SUM(postings).
Refs #19
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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