Operationalize nightly observability, SLOs, alerting, rollback, and revocation

[litroc]

Context

Implements LI-129, MLX-100, and REL-30.

Scope

Report candidate lifecycle and lead time, alert on failed or silent nightlies, and exercise rollback, revocation, and fix-release procedures.

Acceptance criteria

• Regular candidates start within 24 hours; attested critical security fixes are prioritized immediately.
• No candidate remains blocked without visible status and owner.
• Nightly outage, silent blocking, and finalizer failure alert.
• Infrastructure retries are bounded and classified.
• Rollback, revocation, and fix-release exercises are evidenced.
• Dashboards, alerts, and exercise evidence are linked to LI-129.