Add JWKS/OIDC metrics to home page with feature flag configuration

[Copilot]

Adds aggregate statistics computed from jwks-observer key data to the home page, with a data/metrics.yaml config file for toggling each metric independently.

Metrics added

• Catalog Overview — total services, OIDC vs JWKS-only
• Key Status Overview — active vs inactive key counts
• Algorithm Distribution — RS256, ES256, RSA-OAEP, etc.
• Key Type Distribution — RSA, EC, OKP, oct
• Key Length Distribution — bit-length breakdown
• Key Usage Distribution — sig vs enc
• Elliptic Curve Distribution — P-256, P-384, Ed25519, etc.
• Key Rotation Statistics — rotated key count, avg/min/max lifetime in days

Configuration

data/metrics.yaml controls which sections render. If the file is missing, all metrics default to enabled.

metrics:
  catalog_overview: true
  key_type_distribution: true
  algorithm_distribution: true
  key_length_distribution: false  # disable this one
  # ...

Changes

• data/metrics.yaml — new feature flag config
• main.go — MetricsConfig, CatalogMetrics, computeMetrics() to aggregate key data across all services; loadAllKeyRecords() for unlimited aggregation (vs the per-service display limit); home template now receives HomePageData with metrics
• templates/home.html — conditional metric sections gated on config flags and observer data availability
• assets/style.css — .metrics-grid, .metric-box, .metrics-table with mobile responsive variants

Observer-dependent sections gracefully hide when JWKS_OBSERVER_PATH is unset. Catalog overview (service counts) always renders.

Screenshots

Catalog Overview & Key Status

Algorithm Distribution

Key Length Distribution

Key Rotation Statistics

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