fix(backend): make fee and share calculations use one canonical precision policy

[arisu6804]

Problem

Different code paths can round amounts differently, causing preview values to disagree with executed deposits or withdrawals at boundary values.

Objective

Deliver a production-quality improvement to fee, share conversion, and preview/execution calculations that creates measurable value for correctness, security, reliability, performance, or maintainability.

Implementation scope

• Introduce one documented decimal and rounding policy used by preview, validation, and execution.
• Define fee caps, minimum units, and behavior for zero, near-limit, and unsupported precision inputs.
• Return enough metadata for clients to explain rounding without exposing internal implementation details.

Acceptance criteria

• Preview and execution produce the same canonical result for identical inputs.
• No calculation overflows or silently loses value at supported boundaries.
• Fee and rounding behavior is covered by shared fixtures.

Required validation

• Boundary, property-based, and API contract tests comparing preview and execution results.
• 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 on this issue and explain design tradeoffs.
• Include test evidence, compatibility impact, and migration or rollout notes where relevant.
• 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.