Implement user login with password hash verification and username lookup

internal/auth/password.go

@@ -7,4 +7,9 @@ import (
 func HashPassword(password string) (string, error) {
 	bytes, err := bcrypt.GenerateFromPassword([]byte(password), bcrypt.DefaultCost)
 	return string(bytes), err
+}
+
+func CheckPasswordHash(password, hash string) bool {
+	err := bcrypt.CompareHashAndPassword([]byte(hash), []byte(password))
+	return err == nil
 }