Service tests hit create, update, missing-id, blank-collapses-to-null,
and the snapshot copying behaviour. Route tests hit both pages'
create flows, the budget notes endpoint, and the month update route.
A dedicated assertion confirms that changing notes on a month entry
does not flip the deviation state: notes are free-form annotation,
not a signal of plan drift.
Refs #13
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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