feat(observability): publish standalone Dispatch dashboard and alerts #1255

Why

Make Dispatch's state/alert catalogue from design #1237 / PR #1239 operational without assuming Courier, one worker identity or a universal dashboard backend.

Work

• Package an opt-in dashboard and executable evaluator/rule integration for the chosen compatible metrics backend; separate portable state/alert definitions from optional Prometheus/VictoriaMetrics assets, OTel transport, and deployment-owned notification routing.
• Consume the bounded queue/ownership/execution/export metrics implemented in #1252. Standalone views for eligible/assigned/claimed/executing, PR-fix hand-out/ack/reclaim, blocked human vs operational park, grooming/sync freshness, and known/unknown worker availability.
• Implement the correct identity/age-based semantics for stale assignments and overdue unacked hand-outs: continuous nonzero aggregate backlog during healthy throughput does not imply a stranded item.
• For windowed DB failure gauges, use the current in-window counts rather than delta/counter-style operations that miss event replacement. A4 pipeline-idle alerts must exclude groomer/heartbeat emissions and be gated by fresh upstream sync.
• Missing first successful sync, absent scrape, unsupported worker ownership and no meaningful traffic are distinct cases; healthy idle is not a page. Define exact thresholds, evidence, suppression and operator actions.
• Keep UI/asset configuration independent of home-ops, worker credentials, courier metrics, Grafana provisioning or a mandatory Alertmanager.

Acceptance

• Replayable fixtures or rule tests cover aged stranded claim, overlapping rapidly resolved assignments (no alert), prolonged unacked hand-out, zero-actionable queue, first-sync missing, failed sync, real execution outage, audit failures in a constant-size rolling window, and stopped telemetry.
• A non-Courier consumer and completely idle Dispatch both render accurately. No privilege escalation or autonomous claim release via dashboards.
• Document optional backend/evaluator matrix and installation guidance separately from upstream semantics.

Dependencies

Blocked by #1252 and accepted #1239. Do not conflate dashboard work with the PR-fix/ownership reaper implementation.