Add Stellar SEP-0010 web authentication

[Mystery-CLI]

🔧 Title: Add Stellar SEP-0010 web authentication

📘 Description
SEP-0010 is the Stellar standard for web authentication using a Stellar keypair. Implementing it allows users to authenticate with their Stellar private key instead of a username/password, which is the standard approach for Stellar-native apps.

✅ Acceptance Criteria

• Implement GET /api/auth/stellar/challenge (returns a challenge transaction)
• Implement POST /api/auth/stellar/token (verifies signed challenge, returns JWT)
• Add a "Sign in with Stellar" button to the login UI
• Follow the SEP-0010 specification exactly
• Add tests for the full SEP-0010 flow

🔧 Context: backend/src/routes/auth.js; Stellar SEP-0010 spec.

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