security: enforce server-side registration string length and complexity filters (#345)

[prakshithamalla-art]

Closes #345

Changes Made:

• Registration Input Hardening: Injected a localized security verification gate directly into the initial sequence parameters of the signup function inside backend/src/controllers/auth.controller.js.
• Password Complexity Enforcement: Implemented regex pattern matching constraints (complexityRegex) requiring incoming password fields to strictly contain a minimum threshold of 8 characters, structured with alternating uppercase and lowercase characters, standard digits, and supported special syntax elements.
• Upstream Validation Drop: Configured standard HTTP 400 Bad Request error returns with contextual error strings to drop weak requests immediately, mitigating downstream computational overhead on intensive bcrypt.hash operational tasks.
• Maintained Structural Integrity: Preserved original code logic structures across all neighboring authentication and profile mapping routers.

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• backend/src/controllers/auth.controller.js

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[prakshithamalla-art]

@UTKARSHH20 pls check it and merge it