feat(security): add password complexity validator

- Implement strict password requirements:
  - Minimum 8 characters
  - At least one uppercase letter
  - At least one lowercase letter
  - At least one number
  - At least one special character
- Add custom validator for Echo integration
- Add GetPasswordRequirements helper function
- Add ValidatePassword function for manual validation

internal/middleware/password_validator.go

@@ -0,0 +1,96 @@
+package middleware
+
+import (
+	"fmt"
+	"regexp"
+
+	"github.com/go-playground/validator/v10"
+)
+
+// PasswordValidator validates password complexity requirements
+type PasswordValidator struct{}
+
+// Validate checks if a password meets complexity requirements:
+// - Minimum 8 characters
+// - At least one uppercase letter
+// - At least one lowercase letter
+// - At least one number
+// - At least one special character
+func (v *PasswordValidator) Validate(fl validator.FieldLevel) bool {
+	password := fl.Field().String()
+
+	// Check minimum length
+	if len(password) < 8 {
+		return false
+	}
+
+	// Check for uppercase
+	hasUpper := regexp.MustCompile(`[A-Z]`).MatchString(password)
+	if !hasUpper {
+		return false
+	}
+
+	// Check for lowercase
+	hasLower := regexp.MustCompile(`[a-z]`).MatchString(password)
+	if !hasLower {
+		return false
+	}
+
+	// Check for number
+	hasNumber := regexp.MustCompile(`[0-9]`).MatchString(password)
+	if !hasNumber {
+		return false
+	}
+
+	// Check for special character
+	hasSpecial := regexp.MustCompile(`[!@#$%^&*()_+\-=\[\]{};':"\\|,.<>\/?]`).MatchString(password)
+	if !hasSpecial {
+		return false
+	}
+
+	return true
+}
+
+// GetPasswordRequirements returns a human-readable list of password requirements
+func GetPasswordRequirements() []string {
+	return []string{
+		"At least 8 characters long",
+		"At least one uppercase letter (A-Z)",
+		"At least one lowercase letter (a-z)",
+		"At least one number (0-9)",
+		"At least one special character (!@#$%^&*()_+-=[]{}|;':\",./<>?)",
+	}
+}
+
+// ValidatePassword checks a password and returns an error if it doesn't meet requirements
+func ValidatePassword(password string) error {
+	if len(password) < 8 {
+		return fmt.Errorf("password must be at least 8 characters long")
+	}
+
+	if !regexp.MustCompile(`[A-Z]`).MatchString(password) {
+		return fmt.Errorf("password must contain at least one uppercase letter")
+	}
+
+	if !regexp.MustCompile(`[a-z]`).MatchString(password) {
+		return fmt.Errorf("password must contain at least one lowercase letter")
+	}
+
+	if !regexp.MustCompile(`[0-9]`).MatchString(password) {
+		return fmt.Errorf("password must contain at least one number")
+	}
+
+	if !regexp.MustCompile(`[!@#$%^&*()_+\-=\[\]{};':"\\|,.<>\/?]`).MatchString(password) {
+		return fmt.Errorf("password must contain at least one special character")
+	}
+
+	return nil
+}
+
+// RegisterPasswordValidation registers the password validator with the validator instance
+func RegisterPasswordValidation(v *validator.Validate) error {
+	return v.RegisterValidation("passwordcomplex", func(fl validator.FieldLevel) bool {
+		pv := &PasswordValidator{}
+		return pv.Validate(fl)
+	})
+}