DHT: Implement encrypted document storage API

[umwelt]

Context

After schema design is complete, implement the core DHT storage API for encrypted identity documents. This fulfills the Phase 1 deliverable: "Integrate DHT file storage layer".

Scope

• Implement DHT put operation (store encrypted document)
• Implement DHT get operation (retrieve by DID)
• Handle ChaCha20-Poly1305 encryption/decryption
• Implement replication logic (3+ nodes)
• Add error handling for network failures
• Public API: store_identity(did: &str, document: &[u8]) -> Result<()>
• Public API: retrieve_identity(did: &str) -> Result<Vec<u8>>

Dependencies

• Issue DHT: Design storage schema for identity documents #1: DHT schema design must be complete

Acceptance Criteria

• store_identity function implemented and working
• retrieve_identity function implemented and working
• Documents encrypted with ChaCha20-Poly1305 before storage
• Minimum 3-node replication enforced
• Network error handling with retries
• Unit tests with >80% coverage
• Integration tests with local DHT cluster
• API documentation with examples

Technical Notes

• Use lib-crypto for ChaCha20-Poly1305 encryption (lib-crypto/src/symmetric/chacha20.rs)
• Implement exponential backoff for retries (max 3 attempts)
• Store metadata (timestamp, version) with encrypted document
• Reference lib-identity/src/backup/crypto.rs for existing ChaCha20-Poly1305 usage patterns