Pydantic v2's model_dump(mode="json") serializes Decimal as strings (e.g.,
"33.33" instead of 33.33), causing frontend crashes when calling .toFixed()
on string values. Introduced FloatDecimal type alias with PlainSerializer
to ensure Decimal fields are serialized as floats in JSON responses.
Also fixed frontend Transaction interface to match backend field names
(created_at → executed_at, transaction_type → tx_type).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
