DHT Integration: Wire DHT storage to identity API

[umwelt]

Context

After lib-network DHT storage is implemented, integrate it into zhtp identity API endpoints for distributed storage. This fulfills the Phase 1 deliverable: "Integrate DHT file storage layer".

Scope

• Update POST /api/v1/identity/create to store in DHT
• Update GET /api/v1/identity/{did} to retrieve from DHT
• Add DHT health check endpoint
• Implement fallback to local storage if DHT unavailable
• Async replication (don't block API responses)
• Add storage location to API responses ("dht" or "local")

Dependencies

• lib-network Issue DHT Integration: Wire DHT storage to identity API #2: DHT storage API must be complete

Acceptance Criteria

• Identity creation stores in DHT asynchronously
• Identity retrieval checks DHT first, falls back to local
• Health check endpoint: GET /api/v1/system/dht/health
• API response includes storage_location field
• Timeout handling (3s max for DHT operations)
• Graceful degradation if DHT down
• Integration tests with local DHT cluster
• API documentation updated
• Monitoring logs for DHT success/failure rates

Technical Notes

• Use async/await for non-blocking DHT operations
• Cache identity locally after DHT retrieval
• Consider eventual consistency (DHT write may take time to replicate)
• Core SID requirement: "Storage: Encrypted and distributed across a DHT"
• Existing identity API: zhtp/zhtp/src/api/handlers/identity/mod.rs