[CT-03] Implement POST /transfer in Rust verifier — record ownership transfer on Stellar

[mftee]

Overview

POST /transfer in the Rust verifier is a stub (returns 200 with empty body or a placeholder). Ownership transfer is a key workflow for land documents — when a parcel changes hands, the new ownership must be recorded permanently on the Stellar blockchain.

Background

File: contract/src/lib.rs — transfer_handler() (currently a stub)

Transfer approach on Stellar:
Write a new ManageData entry recording the transfer event:

• Key: "transfer_" + &hash[..54] (max 64 bytes)
• Value: { fromPublicKey, toPublicKey, transferredAt } serialized to bytes

This creates an immutable, ordered record of all transfers. Each call adds a new entry (with a monotonic counter suffix if multiple transfers occur).

Request body:

{
  "hash": "sha256hash...",
  "fromPublicKey": "GXXX...",
  "toPublicKey": "GYYY...",
  "reason": "sale"
}

Response: { txHash, ledger, transferredAt }

Acceptance Criteria

• POST /transfer writes a transfer ManageData entry on Stellar
• Transfer key format: "transfer_" + hash[:54]
• Returns { txHash, ledger, transferredAt } on success
• Redis cache for the document hash is updated to include the latest owner's public key
• GET /verify/:hash/history returns all transfer records for this document in chronological order
• Returns 404 if the hash has no prior anchor record (cannot transfer what was never anchored)