[Feature] Report gateway readiness separately from heartbeat liveness in Admin #7718

Goal

Let Admin distinguish a recently reporting gateway from one ready to serve traffic. Current instance status handling derives ONLINE from heartbeat arrival/age. It does not report startup completion, readiness reason or draining status.

The gateway already has an opt-in WebSocket initial-sync readiness indicator. Reuse established readiness semantics instead of inventing a second contradictory health check.

Scope

• Reuse existing status-reporting channels and define a backward-compatible, versioned optional status extension.
• Report readiness independently of liveness, with observation time, a bounded reason code and supported capability indicators. Missing/old data means unknown, not ready.
• Define stable deployment instance identity and per-start boot identity so reports from a prior process cannot overwrite the current observation.
• Distinguish configuration channel connectivity from readiness and allow an explicit serving/draining state if supported by the runtime.
• In Admin list/detail show liveness, readiness, last observation age and reason separately. Keep legacy numeric status compatible.

Acceptance criteria

• Initializing, ready, readiness-failed and stale observations are distinguishable while heartbeat stays healthy.
• Reconcile with Actuator and initial synchronization behavior; do not claim that initial sync completion alone proves every dependency healthy.
• Test legacy reporters, unknown fields, incompatible report versions, restart identity, out-of-order observations and namespace isolation.
• Test two Admin nodes and failover against a documented authoritative status view.
• Telemetry/reporting failure does not break gateway request processing; payload size/cadence and diagnostic content are bounded.
• Coordinate dashboard changes, schema/API compatibility and documentation.

Related: #7692/#7693/#7694/#7696/#7697 are existing status correctness fixes. #7652 adds local file mode: it should be able to report local startup readiness without falsely requiring Admin synchronization.