feat(backend): centralize amount and currency validation across API and service layers

[arisu6804]

Problem

Different layers can accept different decimal precision, unsupported currencies, or amounts that later fail settlement.

Objective

Deliver a production-quality improvement to money amounts, currencies, and transfer validation that creates measurable value for correctness, security, reliability, performance, or maintainability.

Implementation scope

• Define canonical currency metadata, minor units, rounding, minimum/maximum values, and validation errors in one policy.

Acceptance criteria

• API validation and settlement use the same canonical amount; unsupported currency and overflow fail before mutation; preview matches execution.

Required validation

• Currency matrix, precision, overflow, boundary, and preview/settlement contract tests.
• Existing tests and CI remain passing.
• Add regression coverage for the original failure mode.
• Do not weaken, delete, or skip unrelated tests to obtain a green build.

PR quality bar

• Keep the PR focused and explain design tradeoffs, compatibility impact, and test evidence.
• Avoid typo-only, documentation-only, cosmetic-only, or unrelated refactor submissions.

Out of scope

• Broad rewrites not required by the acceptance criteria.
• Changes to unrelated services, contracts, or user flows.