Executive dashboard: add compliance and audit-readiness reporting signals

[FrodeHus]

Goal

Expose governance posture for executives and audit committees: SLA adherence by policy, audit trail completeness, evidence coverage, policy exceptions, and board-report-ready summary status.

Current state

• PatchHound has audit logging, tenant SLA configuration, remediation decisions, approval tasks, and an AI/deterministic executive briefing.
• Dashboard summary currently includes SLA compliance percent and trends, but not audit/evidence completeness or compliance-ready indicators.

Scope

• Add governance summary fields for SLA adherence by severity/policy, evidence coverage for accepted risk/remediation decisions, overdue reviews, and audit trail completeness.
• Include whether the latest executive briefing is fresh, deterministic, or AI-generated.
• Surface a compact compliance/governance panel in the executive dashboard.

Acceptance criteria

• Dashboard API returns SLA adherence by severity or configured policy bucket.
• Accepted-risk/remediation evidence coverage is measurable once evidence fields exist.
• Briefing freshness and generation mode are visible.
• Tests cover SLA policy rollups and missing evidence/audit metadata.