Add input sanitization middleware to prevent stored XSS attacks

[ramya2006sri]

Description

This PR adds centralized input sanitization middleware to help prevent stored XSS attacks by sanitizing user-provided text before it is stored in the database.

Changes Made

• Added a reusable sanitizeInput middleware using sanitize-html.
• Applied sanitization to product creation and update endpoints.
• Applied sanitization to review creation and update endpoints.
• Applied sanitization to user registration.
• Applied sanitization to newsletter subscription.
• Added the required sanitize-html dependency.

Security Impact

Previously, user-supplied text fields were stored without sanitization, which could lead to stored XSS if the data is later rendered by API consumers, admin dashboards, or future server-side rendered clients.

This change sanitizes incoming text input by removing HTML tags and attributes before persistence while preserving the existing API behavior.

Type of Change

• Security improvement
• Bug fix (non-breaking change)

Testing

• Verified middleware integration on the updated routes.
• Confirmed backend compiles after the changes.
• No breaking changes to existing API endpoints.

Closes #325

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