Function isStrongPassword

Check if a password meets defined strength requirements:

• At least one uppercase letter
• At least one lowercase letter
• At least one digit
• At least one non-alphanumeric symbol
• Minimum length of 12 characters

This policy is applied consistently to:

• local bootstrap admin (CREATE_ADMIN=true)
• user self-registration (registerUser)
• model-level persistence validation (defense in depth)