Fix/issue 2357 backend password complexity

[Prateek2007-cmd]

Description

Resolves Issue #2357 (Missing Backend Password Complexity Validation).

The UserRegister validation layer previously suffered from a dangerous "Client-Side Trust" vulnerability. While the frontend rigorously verified password composition, the backend Pydantic model (password_complexity) only checked for a single digit and an uppercase character. This allowed malicious actors bypassing the UI to easily register weak, brute-forceable passwords.

This PR establishes zero-trust backend enforcement of cryptographic composition rules.

Changes Made

• Strict Composition Policy: Extended the password_complexity validator in backend/app/schemas.py to mandate the presence of lowercase letters [a-z] and special characters [@$!%*?&].
• API Hardening: Any automated request attempting to register a non-compliant password will now reliably hit a 422 Unprocessable Entity response, successfully stopping brute-force dictionary attacks.

Type of Change

• Security Fix
• Data Validation

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