Oracle-Based Invoice Verification to Prevent Fake Invoice Fraud

[samjay8]

Title: feat: Oracle-Based Invoice Verification to Prevent Fake Invoice Fraud

Labels: backend, contracts, compliance, security, priority-high

Description:
The biggest risk in invoice financing is fake or duplicate invoices.
Before any invoice appears on the marketplace, an authorized oracle
or admin must verify its legitimacy. This issue adds a mandatory
verification gate between invoice creation and marketplace listing.

What Needs to Be Done:

Backend:

• InvoiceVerification entity:
  id (uuid), invoice_id (FK), verifier_id (FK to User),
  status (ENUM: PENDING/APPROVED/REJECTED), notes,
  document_hash (sha256 of attached doc), verified_at
• POST /admin/invoices/:id/verify
  • Admin or oracle role only
  • Approves invoice — allows transition to SUBMITTED
  • Emits invoice.verified webhook event
• POST /admin/invoices/:id/reject-verification
  • Admin rejects with required reason
  • Invoice stays in DRAFT with rejection_reason set
  • Emits invoice.rejected webhook event
• Verification gate:
  Invoice can only be submitted on-chain after APPROVED verification
  POST /invoices/:id/submit returns 403 if not verified
• Duplicate detection:
  SHA-256 hash of invoice data checked against existing records
  Duplicate hash rejected with 409

Smart Contract (contracts/invoice_registry/src/lib.rs):

• Add oracle_address to contract storage
• set_oracle(admin, oracle_address) — admin sets oracle
• register_invoice() requires either business + oracle co-signature
  or proof of oracle approval

Key Files:

• apps/backend/src/verification/verification.module.ts (new)
• apps/backend/src/verification/verification.service.ts (new)
• apps/backend/src/verification/verification.controller.ts (new)
• apps/backend/src/verification/verification.entity.ts (new)
• contracts/invoice_registry/src/lib.rs (update)

Acceptance Criteria:

• Unverified invoices cannot be submitted on-chain
• Unverified invoices do not appear in marketplace
• Duplicate invoice hashes rejected with 409
• Admin can approve or reject with reason
• Webhook fired on every verification status change
• Oracle address configurable in contract storage
• Unit tests cover: approve, reject, duplicate detection,
  submit gate, unauthorized verifier

Security Notes:

• Verification is a one-way gate — no downgrade from approved
• Oracle signature verified on-chain
• Document hash stored for tamper detection

Branch: feat/invoice-verification-oracle

Commit: feat(verification): add oracle-based invoice verification gate before marketplace listing