feat(did): complete DID document management and VC issuance endpoints

[devwums]

Summary

Completes the DID & Verifiable Credentials system by wiring up the missing controller endpoints and building W3C-compliant DID document resolution.

Changes

did.controller.ts

• POST /api/did/register - register a new DID with its secp256k1 public key and return a W3C DID document
• GET /api/did/document/:did - resolve any registered DID to its W3C Core-compliant document
• POST /api/did/credentials/issue - issue a KYC verifiable credential (delegates to VcIssuerService)
• PATCH /api/did/credentials/:id/revoke - revoke an active credential
• Grouped endpoints by concern (DID document management / authentication / credentials / ZKP)

did-auth.service.ts

egisterDid(did, publicKey) - persists the DID document, throws ConflictException on duplicate registration

esolveDidDocument(did) - returns
ull if the DID is unknown (controller maps this to 404)

• �uildW3cDocument() - private helper that constructs the W3C DID document with @context, �erificationMethod, �uthentication, and �ssertionMethod fields (EcdsaSecp256k1VerificationKey2019)

Acceptance criteria addressed

• ? Users can create and manage DID identities (POST /register, GET /document/:did)
• ? KYC credentials are issued as verifiable credentials (POST /credentials/issue)
• ? ZKP verification enables privacy-preserving checks (existing POST /verify-proof, unchanged)
• ? DID-based authentication works alongside traditional JWT (existing challenge/verify flow, unchanged)
• ? DID documents are available on standard resolvers (W3C DID Core format returned by GET /document/:did)

closes #399