Backend: Add Input Sanitization for User Memos

[Abdulazeem-code]

Description: Ensure that user-provided memos during registration do not contain malicious payloads (XSS protection).

Acceptance Criteria:

• Memos are stripped of HTML/script tags before being saved to the database.

Branching Instructions: Create branch fix/memo-sanitization from main.

[ojuotimi932]

@ojuotimi932 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

I have reviewed the description and I'm confident I can resolve this
Kindly assign

ℹ️ Repo Maintainers: To accept this application, review their application or assign @ojuotimi932 to this issue.

[ZuLu0890]

@ZuLu0890 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

I would love to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @ZuLu0890 to this issue.

[Caneryy]

@Caneryy has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hello, may I work on this issue?

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

[drips-wave]

Congratulations, @ojuotimi932! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

🧑‍💻 @ojuotimi932: 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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