Service tests assert that create_month produces origin fields matching
the budget, that edits flip deviation_state to edited, that added rows
are new_in_month, and that a budget entry deleted after snapshot leaves
the month entry unchanged. Route tests exercise the create flow,
applied updates, name edits producing the modified tag, per-month
target isolation, and the malformed-year-month 404.
Refs #3
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
