🛡️ Sentinel: [HIGH] Fix XSS and DoS in Socket.IO handlers

[google-labs-jules]

This PR addresses a high-severity security vulnerability in the Socket.IO event handlers where user input was not properly sanitized or validated.

Changes:

1. Created backend/utils/security.js: Added a dedicated utility for HTML escaping to prevent XSS.
2. Updated backend/server.js:
   • Imported the escapeHtml utility.
   • Added input validation to send_message and guild_message handlers to ensure message is a string and does not exceed 1000 characters.
   • Applied escapeHtml to the message content before emitting it to other users.
3. Added Tests: Created backend/tests/security/socket_sanitization.test.js to verify:
   • XSS payloads (e.g., <script>) are correctly escaped.
   • Non-string inputs are rejected.
   • Oversized messages (>1000 chars) are rejected.

Verification:
Run cd backend && npm test tests/security/socket_sanitization.test.js. All tests should pass.

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PR created automatically by Jules for task 13127777788474757747 started by @4444J99

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[4444J99]

The genuine security want here has been distilled onto main and merged as #158 (bfcd3836):

• Token-refresh privilege escalation — authController.refreshToken now enforces the same user.status !== 'active' gate as login, so a suspended/inactive/deleted user can no longer mint fresh access tokens.
• Socket.IO message XSS / input validation — new backend/utils/sanitize.js (HTML-escape + control-char strip + length cap + type check) is wired into both the send_message and guild_message handlers.

Both are covered by tests (refreshTokenStatus.test.js, messageSanitize.test.js). Keeping this PR open per our never-close doctrine — the fix now lives on main. If your PR targets a distinct vector not covered above, reply here and it becomes the next distillation.