core: build full Stellar event indexer service

[EmeditWeb]

Problem

The API has no way to track on-chain events from
StepFi contracts. When a loan is approved on-chain,
the API does not know.

What To Build

1. Create src/indexer/ module
2. Use BullMQ for event processing queue
3. Horizon event polling every 6 seconds
4. Create indexer_state table in Supabase
5. Add health check GET /indexer/status

Files To Touch

• src/indexer/ (new module)
• src/app.module.ts
• supabase/migrations/[ts]_indexer_state.sql

Acceptance Criteria

• Indexer starts automatically on API boot
• All 5 contract event types handled
• Events processed idempotently
• Last processed ledger persisted
• npm run build passes

Mandatory Checks Before PR

• npm run build passes
• No any types
• All event handlers tested
• PR references this issue

[Dev-ElvisTobi]

I would love to work on this issue

[EneGab]

I can work on this issue and deliver results ASAP, plese assign me

[Adeyemi-cmd]

hello maintainer. i would love to work. on this

[khaylebfortune]

Hello Maintainer, I can fix this issue

[grantfox-oss]

🦊 GrantFox — @khaylebfortune 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 #24)
2. Your PR will be reviewed by the StepFi-app 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 campaign!

@khaylebfortune's PR #50 was approved and merged by @EmeditWeb.

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

👏 Great work, @khaylebfortune! Keep contributing to StepFi-app.