docs: Add production operations runbook

[Dahkenangnon]

Problem

Create comprehensive operational documentation for production deployments in docs/runbooks/.

Proposed shape

1. Key Rotation (docs/runbooks/key-rotation.md)

• Step-by-step JWKS rollover procedure
• Key state transitions (pending → active → retiring → revoked)
• Using /federation_historical_keys endpoint
• Timeline recommendations
• Rollback procedures

2. Trust Anchor Migration (docs/runbooks/trust-anchor-migration.md)

• Updating authority_hints in Entity Configuration
• Re-resolving trust chains
• Coordination with superior authorities
• Zero-downtime migration strategies

3. Cache Invalidation (docs/runbooks/cache-invalidation.md)

• When to invalidate cached Entity Configurations
• Trust Chain cache expiration policies
• Cache warming strategies
• Manual cache clearing procedures

4. Trust Mark Revocation (docs/runbooks/trust-mark-revocation.md)

• Immediate revocation procedures
• Updating Trust Mark Status endpoint
• Notifying affected parties
• Audit trail requirements

5. Monitoring and Metrics (docs/runbooks/monitoring.md)

• Key metrics to track:
  • Trust Chain resolution success/failure rates
  • Cache hit rates
  • Entity Statement expiration events
  • Federation endpoint response times
• Alerting thresholds
• Dashboard examples

6. Failure Modes (docs/runbooks/failure-modes.md)

• Authority unavailable scenarios
• Expired Entity Statement handling
• Clock skew detection and mitigation
• Network timeout recovery
• Conflicting trust path resolution

Format

Each runbook should include:

• Problem description
• Step-by-step procedures
• Code examples where applicable
• Troubleshooting guide
• Related endpoints/APIs

Alternatives considered (optional)

No response