security(contract): add governed admin rotation and cooldown policy

[arisu6804]

Problem

Immediate admin changes make compromised or mistaken rotations difficult to detect or cancel.

Objective

Deliver a production-quality improvement to administrator and governance lifecycle that creates measurable value for correctness, security, reliability, performance, or maintainability.

Implementation scope

• Implement propose/accept/cancel with a cooldown and expiry; restrict callers; emit auditable events; prevent replay.

Acceptance criteria

• Only the pending admin can accept after cooldown; expired proposals fail; cancellation and active authority are deterministic.

Required validation

• Authorization, cooldown, expiry, replay, cancellation, and event 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.