[GREEN] Add UserRepository interface for user data access with email lookup

src/main/java/com/safetypin/authentication/repository/UserRepository.java

+package com.safetypin.authentication.repository;
+
+import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.stereotype.Repository;
+import com.safetypin.authentication.model.User;
+
+@Repository
+public interface UserRepository extends JpaRepository<User, Long> {
+    User findByEmail(String email);
+}