feat(verifier): implement POST /transfer and GET /verify/:hash/history on Stellar

[mijinummi]

📝 Summary

Implements POST /transfer and GET /verify/:hash/history in the Rust verifier (#856) to record land document ownership transfers permanently on the Stellar blockchain via ManageData entries and maintain an indexed transfer history.

Closes #856

🛠️ Changes Introduced

• contract/src/models/transfer.rs:
  • Added request/response structs: TransferRequest, TransferPayload, TransferResponse, TransferRecord, and TransferHistoryResponse.
• contract/src/stellar/transfer.rs:
  • Implemented key generation helper build_transfer_key() ensuring keys comply with Stellar's 64-byte ManageData limits (transfer_ prefix for initial, tr_ + counter suffix for subsequent transfers).
  • Added record_transfer_on_stellar() to serialize transfer payloads and record ManageData transactions on Stellar.
• contract/src/handlers/transfer.rs:
  • POST /transfer: Validates document anchor existence in Redis (returns 404 if unanchored), writes transfer record on Stellar, updates the cached owner in Redis, and increments the document's monotonic transfer counter.
  • GET /verify/:hash/history: Validates anchor existence and aggregates transfer history records sequentially in chronological order.
• contract/src/lib.rs:
  • Wired /transfer and /verify/{hash}/history routes into the Actix web service configuration.

---

🧪 How to Test

1. Verify Unanchored Transfer Rejection (404):

   curl -X POST http://localhost:8080/transfer \
     -H "Content-Type: application/json" \
     -d '{
       "hash": "nonexistent_hash_123",
       "fromPublicKey": "GXXX...",
       "toPublicKey": "GYYY...",
       "reason": "sale"
     }'

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