feat(backend): add transactional reconciliation for vault accounting invariants

[arisu6804]

Problem

Balances, shares, and transaction records can drift if a multi-record mutation partially succeeds or a downstream write fails.

Objective

Deliver a production-quality improvement to storage, vault accounting, and analytics that creates measurable value for correctness, security, reliability, performance, or maintainability.

Implementation scope

• Define the accounting invariants for assets, shares, fees, and transaction status.
• Execute related writes atomically where supported and record recoverable reconciliation findings when not.
• Expose a bounded reconciliation report that identifies mismatches without silently changing financial state.

Acceptance criteria

• Failed mutations leave no partial accounting state.
• Reconciliation detects seeded mismatches with actionable identifiers.
• The report is authenticated, bounded, and never performs an implicit repair.

Required validation

• Rollback/failure injection tests, invariant tests, and end-to-end reconciliation tests with representative data.
• 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.