fix(backend): centralize streaming amount precision and boundary rules

[arisu6804]

Problem

Separate calculations can disagree on streamed amounts, rounding, minimum units, or final withdrawal values.

Objective

Deliver a production-quality improvement to vesting projections, balances, and amount validation that creates measurable value for correctness, security, reliability, performance, or maintainability.

Implementation scope

• Introduce canonical fixed-point math and rounding; define time and amount units; use the policy in previews, validation, and settlement.

Acceptance criteria

• Preview equals execution for identical state; unsupported precision and overflow fail safely; no value is created or lost by rounding.

Required validation

• Boundary, property, preview/settlement, and long-duration 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.