[BUG] Instance state code zero means DELETED in backend but UNKNOWN in dashboard #7694

Current behavior

InstanceStatusEnum defines code 0 as DELETED, and the timeout checker assigns it after a prolonged offline period. The shipped dashboard maps code 0 to SHENYU.INSTANCE.SELECT.STATE.UNKNOWN; database schema comments also describe 0 as unknown. The same timed-out instance consequently has conflicting meanings in list/chart/API displays.

Dashboard evidence at the bundled build revision: https://github.com/apache/shenyu-dashboard/blob/649bd45bd0a1609b09d4b32c9c90b85544515a72/src/routes/System/Instance/index.js#L446

Acceptance criteria

• Define one documented state-code contract shared by backend API, chart labels, dashboard and schema comments.
• Decide whether expired/deleted and unknown require distinct states; preserve stored numeric values or provide an explicit migration.
• Test online/offline/expiry/recovery and unknown/unrecognized values.
• Coordinate main-repo and dashboard changes and synchronize the built UI.
• Do not reinterpret historical data silently.

Source evidence

Reviewed source. Baseline: b4e115367fd57adc8aba332d655b1009932ba394. Report based on source inspection; no runtime reproduction was executed during issue preparation.