Add /api/billing/credits endpoint for prepaid balance

[greatest0fallt1me]

Description

This is a backend issue for the GrantFox campaign. This is a backend issue for the GrantFox campaign. Track prepaid credit balance per developer.

Requirements and Context

• Implement per the description
• Add focused tests for the change
• Document any API/visible changes
• Adhere to repo's lint and code style
• Must be secure, tested, and documented
• Should be efficient and easy to review

Suggested Execution

1. Fork the repo and create a branch

   git checkout -b feature/billing-credits

2. Implement changes
   • src/routes/billing/credits.ts (new)
   • drizzle/migrations/0014_credits.sql (new)
3. Test and commit
   • Run the repo's standard test suite and lint
   • Cover edge cases; include output in the PR

Example commit message

feat: prepaid credits endpoint

Acceptance Criteria

• Implementation matches the description
• Tests added and passing
• Code review approved
• Docs updated

Guidelines

• Minimum 90% test coverage on changed lines
• Input validation at the boundary; standardized error envelope
• Structured logging with correlation IDs
• Clear documentation and inline comments
• Timeframe: 96 hours

[rahimatonize]

@rahimatonize has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

hi maintainer i would love to work on this issue, please asign

ℹ️ Repo Maintainers: To accept this application, review their application or assign @rahimatonize to this issue.

[drips-wave]

Congratulations, @rahimatonize! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @rahimatonize: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊

[grantfox-oss]

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

@rahimatonize's PR #571 was approved and merged by @greatest0fallt1me.

🏆 @rahimatonize: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (541 total points). Track your full progress on GrantFox.

👏 Great work, @rahimatonize! Keep contributing to CalloraOrg.

[drips-wave]

This issue has been completed by @rahimatonize as part of the Stellar Wave Program's 6th Wave 🥳

😎 @rahimatonize: You earned 100 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here. You can also Leave a review to share your experience working on this issue.

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