Wallet-based authentication (Stellar/Soroban)

[Martins0152]

Description
Implement POST /api/v1/auth/wallet-auth allowing a user to authenticate by signing a challenge message with their Stellar wallet, verified against their walletAddress.

Acceptance Criteria

• Server issues a one-time challenge string tied to a wallet address (short expiry, e.g. 5 minutes).
• Server verifies the signed challenge against the Stellar public key before issuing a JWT.
• Reused or expired challenges are rejected.
• If no User exists for that wallet address, a new one is created with role DONOR by default (documented choice - flag if you think it should differ).
• Tests cover valid signature, invalid signature, and expired challenge cases.
• openapi.yaml documents the endpoint including the challenge/response flow.

Suggested reward: $30-$40