No configurable scheme allowlist or length cap for `webhook_url`

[Darkdruce]

Summary

Beyond the http(s) prefix check there is no max length or configurable scheme/host policy for webhook URLs.

Location

• src/api/payments.rs:129-136

Details & impact

Extremely long URLs are stored/echoed, and there's no operator control over allowed destinations (ties into SSRF/HTTPS issues).

Suggested fix

Enforce a max length and an operator-configurable host/scheme allowlist.

Acceptance criteria

• URL length and destination policy are enforced and configurable.

[ayomideaccess]

I've successfully resolved some issues on this project. Permit me to work on this too. I'm already familiar with this project. I'll get it done soon.

[grantfox-oss]

🦊 GrantFox — @ayomideaccess 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 #129)
2. Your PR will be reviewed by the StellarGateLabs maintainers

Good luck! Track your progress on GrantFox.