Models use Money = Annotated[Decimal, PlainSerializer(float)] so storage
is exact NUMERIC(15,2) (rates 15,6) while JSON keeps emitting plain
numbers — the frontend contract is unchanged, pinned by
test_models_serialization.py. Service layers coerce to float at their
boundaries (projection math, rate multipliers, agent tool output, the
savings constants become Decimal) so no naive Decimal/float mixing can
raise. Migration 7f567c8bafdb applied to dev: per-row values and the
table sum byte-identical before/after; endpoints verified live on the
new schema. 59/59 tests green.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
