Add end-to-end encryption for telemedicine chat messages in `messagingService.ts`

[success-OG]

Labels: backend security enhancement

Background
backend/services/messagingService.ts (3 KB) stores chat messages in plaintext. Telemedicine chat between pet owners and vets may contain sensitive health information that should be encrypted at rest.

What "Done" Looks Like

• Messages encrypted client-side before sending using the recipient's public key (stored in the user profile)
• Backend stores ciphertext only — cannot read message content
• Key exchange: use X25519 (Diffie-Hellman) for shared secret, AES-256-GCM for message encryption
• Message decryption happens client-side in telemedicineService.ts
• Key rotation: new keypair generated on device change; old messages remain readable via key history
• Update backend/services/__tests__/messagingService.test.ts to verify ciphertext is stored (not plaintext)

[David-Adegboyega]

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

Hi maintainer, I would like to work on this.

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

[drips-wave]

Congratulations, @solomonokeke012-bot! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @solomonokeke012-bot: 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 🌊