feat(ops): add signal freshness SLOs and alertable audit events

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Parent: #32

Goal

Prevent stale data from silently degrading recommendations.

Requirements

• Define freshness SLOs for registry snapshots, scoring model snapshots, GitHub totals, repo segments, contributor decision packs, bounty data, and signal snapshots.
• Add audit events when data becomes stale, degraded, blocked, or repaired.
• Surface stale/degraded state in readiness, sync status, MCP outputs, and decision packs.
• Avoid noisy public GitHub output.

Acceptance Criteria

• /v1/readiness distinguishes service health from signal freshness.
• MCP responses explain when recommendations are stale or degraded.
• Internal jobs can detect and enqueue repair for stale signals.
• Audit events are structured and secret-safe.

Testing Requirements

This issue must preserve the repo's CI gate:

• npm run test:ci must pass.
• Global coverage must remain at or above 95% for lines, statements, functions, and branches.
• Add or update tests for every new branch, fallback path, sanitizer rule, and regression.
• Add invariant/property-style tests when behavior depends on sorting, gating, public/private boundaries, scoring, queue pressure, or source-upload safety.
• Public GitHub output must be tested against forbidden language: wallet, hotkey, raw trust score, payout, reward estimate, farming, private reviewability, and public score estimate.

Required Test Scenarios

• Fresh state.
• Stale registry.
• Stale decision pack.
• Degraded repo segment.
• Repair queued.
• Audit event redaction.