feat(backend): add dependency-aware readiness and clock drift diagnostics

[arisu6804]

Problem

An instance can accept traffic while its provider is unavailable or its clock drift makes vesting projections unreliable.

Objective

Deliver a production-quality improvement to health checks, chain providers, and time-sensitive calculations that creates measurable value for correctness, security, reliability, performance, or maintainability.

Implementation scope

• Separate liveness/readiness; add bounded provider checks and a safe clock-drift signal; redact dependency details.

Acceptance criteria

• Readiness reflects required dependency state; liveness remains responsive; drift beyond policy is visible and does not hang requests.

Required validation

• Dependency timeout, outage, recovery, drift-boundary, and redaction 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.