feat(timesync): expose Keystone time reference and track robot time-gap status

[05F3759DF]

Context

Axon issue #88 proposes warning when robot wall time differs from Keystone server time. Keystone needs the paired server-side capability so robots can compare against a stable reference and operators can see time-gap status in fleet health views.

Related Axon issue: archebase/axon#88

Scope

• Add a lightweight Keystone time reference endpoint suitable for robot time-gap checks.
• Define response fields that allow clients to account for round-trip time and check reliability.
• Ingest or store latest robot-to-Keystone time-gap status reported by Axon.
• Expose time-gap status, offset, threshold status, and checked_at in device health APIs.
• Surface warning and critical time-gap states in operations dashboard summaries.
• Keep this as detection and reporting only; Keystone should not perform host clock synchronization.

Acceptance Criteria

• Axon can query a Keystone time reference endpoint for time-gap checks.
• Keystone can store or expose the latest time-gap status per robot/device.
• Device health distinguishes normal, warning, critical, stale, and unreliable time checks.
• Dashboard and health APIs can include time-gap anomalies.
• Tests or smoke coverage validate endpoint response shape and time-gap status ingestion.