Add Stellar testnet faucet integration for developer onboarding

[vestor-dev]

New developers need funded testnet accounts to try the marketplace. Add:

(1) POST /api/v1/stellar/fund — accepts a public key, calls the Stellar friendbot to fund the account.
(2) Restrict to testnet only (return 403 on mainnet).
(3) Add rate limiting: 1 request per IP per hour.
(4) Surface a 'Fund with Testnet XLM' button in the frontend when wallet balance is 0.
(5) Document the endpoint in the backend README.

[Hollujay]

Hi @vestor-dev, I would like to work on this issue because I will get it done.

[Bolajiomo99]

Hi team, I'd like to take this on as the final applicant. I can implement the POST /api/v1/stellar/fund endpoint to interface with Stellar's Friendbot, adding strict testnet-only validation and IP-based rate limiting (1/hr). I will also wire up the frontend 'Fund with Testnet XLM' button to trigger when the wallet balance hits 0, and update the backend README. Ready to open a clean PR quickly.

[grantfox-oss]

🦊 GrantFox — @Bolajiomo99 has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #33)
2. Your PR will be reviewed by the CortexRail maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@Bolajiomo99's PR #62 was approved and merged by @vestor-dev.

🏆 @Bolajiomo99: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (48 total points). Track your full progress on GrantFox.

👏 Great work, @Bolajiomo99! Keep contributing to CortexRail.