Consolidate Prisma client usage

[robertocarlous]

Problem

src/stellar/events.ts instantiates its own PrismaClient instead of src/db/index.ts singleton.

Acceptance criteria

• Single shared client; no extra connection pools
• Verify graceful disconnect on shutdown

[Lansa-18]

@Lansa-18 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hello maintainer
Let me work on closing this issue.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Lansa-18 to this issue.

[drips-wave]

Congratulations, @Lansa-18! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

🧑‍💻 @Lansa-18: 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.

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[drips-wave]

This issue has been completed by @Lansa-18 as part of the Stellar Wave Program's 5th Wave 🥳

😎 @Lansa-18: You earned 200 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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