Write `docs/api-reference.md` for the indexer REST API

[Nursca]

Description
Third-party developers building on the ILN ecosystem (bots, dashboards, integrations) need a standalone REST API reference that covers every indexer endpoint without reading the indexer source code.

Requirements and context

• OpenAPI-style documentation for every endpoint: method, path, query parameters, request body, response schema, error codes
• Include curl example for each endpoint
• Cover: /invoices, /invoices/:id, /stats, /stats/history, /reputation/:address, /leaderboard, /events, /health
• Include authentication section (API key header)
• Auto-generate from the indexer's route definitions where possible (e.g. via swagger-jsdoc)

Suggested execution

git checkout -b docs/api-reference

• Create docs/api-reference.md (or generate via OpenAPI)
• Validate examples against the testnet indexer deployment
• Link from indexer README.md

Example commit message
docs: write comprehensive indexer REST API reference

[spartan124]

@spartan124 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi, I would like to work on this. I have professional and open source experience implementing similar solutions across several projects. Kindly assign for quick and efficient resolution.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @spartan124 to this issue.

[gloskull]

Hi, I’d love to work on this issue. I have 4 years of experience across backend, frontend, full-stack, and smart contract development. I understand the task clearly and can deliver a clean, efficient, well-tested solution on time, with strong focus on code quality and maintainability. My PR will be submitted in under 10 hours.

[distributed-nerd]

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

can i work on this

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

[grantfox-oss]

🦊 GrantFox — @distributed-nerd has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #296)
2. Your PR will be reviewed by the Invoice-Liquidity-Network maintainers

Good luck! Track your progress on GrantFox.

[drips-wave]

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

🧑‍💻 @distributed-nerd: 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 🌊