security(backend): enforce token scopes consistently across transfer and admin routes #125

Problem

Inconsistent scope checks across route and service layers can expose transfer data or permit an unauthorized state change.

Objective

Deliver a production-quality improvement to authentication, scopes, and administrative APIs that creates measurable value for correctness, security, reliability, performance, or maintainability.

Implementation scope

• Define action/resource scopes; enforce them at service boundaries; cover direct, list, bulk, and admin paths; prevent identifier enumeration.

Acceptance criteria

• A token can perform only documented actions; unauthorized responses are consistent and non-enumerating; admin paths require explicit scopes.

Required validation

• Scope matrix, cross-account, bulk-route, malformed-ID, and audit authorization 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.