feat(backend): integrate pluggable external KYC providers for SEP-12

[mofejo1]

Closes #235

Summary

• implement a generic IKycProvider interface for third-party KYC integrations
• add provider adapters for mock, persona, and shufti
• add config-driven provider selection via KYC_PROVIDER
• add webhook signature validation + normalized webhook event parsing in provider layer
• wire SEP-12 controller to submit through selected provider and persist provider metadata
• reconcile async webhook updates by provider reference first, then by account fallback
• add Prisma schema support for provider metadata (provider, providerRef) + migration

Security

• webhook signatures are validated before processing status updates
• provider callbacks update only matched KYC records
• unknown/invalid webhook payloads are rejected

Tests

• npx jest --coverage=false src/services/kyc-provider.service.test.ts src/api/controllers/sep12.controller.test.ts src/services/kyc.service.test.ts

Notes

• This PR keeps existing SEP-12 flows intact while enabling anchors to select Persona/Shufti/Mock by configuration.

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