feat: add GET /loans/stats endpoint for protocol metrics

[EmeditWeb]

Problem

There is no endpoint returning protocol-wide loan
statistics. The web app explorer page needs this
to show total loans, active loans, repaid loans,
and total volume.

What To Build

Add GET /api/v1/loans/stats endpoint.
No auth required.

Response:
{
totalLoans: number,
activeLoans: number,
repaidLoans: number,
defaultedLoans: number,
totalVolume: number,
onTimeRepaymentRate: number
}

Query Supabase loans table for aggregated counts.

Files To Touch

• src/modules/loans/loans.controller.ts
• src/modules/loans/loans.service.ts

Acceptance Criteria

• Endpoint returns correct aggregated stats
• No auth required
• Full Swagger documentation
• Response matches TypeScript interface
• npm run build passes

Mandatory Checks Before PR

• npm run build passes
• Swagger decorator added
• PR references this issue

[Mmesolove]

Hi maintainer,I have read the description and I can resolve this issue, please kindly assign to me.

[grantfox-oss]

🦊 GrantFox — @Mmesolove 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 #20)
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!

@Mmesolove's PR #47 was approved and merged by @EmeditWeb.

🏆 @Mmesolove: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (153 total points). Track your full progress on GrantFox.

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