feat: add credential tab to explorer

[Bushstar]

Summary

• add published credential indexing to search-server by deriving rows from subject DID manifests during refresh
• add published credential storage and query support across SQLite, Postgres, and in-memory adapters
• add public metrics endpoints for schema counts and published credential listing, including direct lookup by credentialDid
• add a new explorer Credentials tab with schema browsing, credential browsing, and full-page credential detail views
• show credential detail from the subject manifest entry so revealed vs redacted publication is visible
• use signature.signed as the published timestamp when available, with fallback to the subject DID document timestamp

Search-Server Changes

• add published_credentials as a derived index populated from public subject manifests
• add typed published credential records and list/count query interfaces
• expose:
  • GET /api/v1/metrics/schemas/published
  • GET /api/v1/metrics/credentials/published
• support filtering published credential rows by:
  • credentialDid
  • schemaDid
  • issuerDid
  • subjectDid
  • revealed

Explorer Changes

• add /credentials route and header tab
• add schema list view with published counts
• add schema-scoped credential list view with pagination
• add full-page credential detail view with back navigation
• show Credential, Subject, Issuer, and Revealed metadata above the JSON viewer

Notes

• existing search-server databases need a rebuild/reindex to populate published_credentials rows for already indexed DIDs
• the published credential dashboard only covers credentials made visible via publication