feat(stellar): signed challenge hardening + verification docs

[abdulrcrtw]

Hardens and documents the Stellar signed-challenge verification flow.

• Adds per-address verify rate-limiting + expired-challenge cleanup
• Adds regression tests for verify rate limiting + expiry cleanup
• Fixes @qyou/stellar-service test script to run all tests under Jest (and disables watchman)
• Adds a short verification doc with rerun + manual curl smoke steps

Closes #401
Closes #402
Closes #403
Closes #398