[Backend] Add input sanitization middleware against XSS and injection attacks

[Penielka]

Overview

Implement comprehensive input sanitization middleware to protect against XSS, SQL injection, and NoSQL injection attacks.

Acceptance Criteria

• HTML entity encoding on all string inputs
• SQL injection prevention (parameterized queries)
• NoSQL injection protection for MongoDB queries
• File upload validation (type, size, content)
• Tests with common attack vectors

Priority: High

Critical security requirement for user-generated content platform.

[barry01-hash]

I would love to work on this issue

[jobbykings]

can i be assigned this

[grantfox-oss]

🦊 GrantFox — @jobbykings has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #265)
2. Your PR will be reviewed by the AetherEdu maintainers

Good luck! Track your progress on GrantFox.