Backend: Add consistent numeric formatting rules in API responses (money fields)

[greatest0fallt1me]

Description

Define how money-like values are represented in API responses (string vs integer minor units), including rounding rules, and enforce it consistently across endpoints to prevent client-side ambiguity.

Requirements and context

• Must be tested and documented.
• Must include a migration note if changing any existing field format.

Suggested execution

git checkout -b chore/api-money-format

Implement changes

• Add shared formatter and update response models.
• Tests asserting formatting for representative endpoints.
• Update OpenAPI schemas and docs.

Test and commit

npm test

Example commit message

docs(api): standardize money field formats across responses

Guidelines

• Timeframe: 96 hours.