Document Revocation Endpoint

[mftee]

Description:

Implement a POST /revoke endpoint that records a document revocation event on the Stellar blockchain. Revocation marks a previously anchored document as no longer valid (e.g., due to fraud or supersession).

Requirements:

• Add a RevokeRequest struct:

  pub struct RevokeRequest {
      pub document_hash: String,
      pub reason: String,
      pub revoked_by: String,
  }

• The Stellar transaction memo must encode the action as a structured string: REVOKE:<hash> (truncated to 28 bytes)
• Before submitting the revocation, verify that the hash exists on-chain by calling verify_hash. If not found, return 404 with an appropriate message
• Return a RevokeResponse with transaction_id and revoked_at
• Invalidate any cached verification results for this hash after revocation

Acceptance Criteria:

• Attempting to revoke an unanchored hash returns 404
• Revocation transaction is submitted with the correct memo format
• Cache entry for the revoked hash is cleared after successful revocation
• Unit tests for memo construction and the pre-revocation existence check