security: enforce IP-based rate limiting on WebSocket handshakes (#346)

[prakshithamalla-art]

Closes #346

Changes Made:

• WebSocket Handshake Interceptor: Implemented an io.use() global middleware block within backend/src/lib/socket.js to inspect and gate all incoming client connection requests prior to server event loop engagement.
• Rolling Window Rate Limiter: Engineered a localized memory map (connectionRates) tracking client IP addresses (x-forwarded-for compatible). The system enforces a strict threshold envelope dropping IPs that attempt more than 5 rapid handshakes within a 10-second rolling window.
• Resource Exhaustion Mitigation: Instantly drops malicious, high-frequency script attacks with a native Socket Error rejection, completely insulating the downstream routing infrastructure and database aggregations from DDoS memory exhaustion.
• Minimal Diffs: The addition perfectly wraps the existing WebRTC and active presence infrastructure logic, leaving existing downstream events unmodified.

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[prakshithamalla-art]

@UTKARSHH20 pls check it and merge it