Add refresh token support with /api/auth/refresh endpoint

- RefreshToken entity added with 1-token-per-user logic.
- JWT can be renewed without full login using refresh token.

src/main/java/com/skycrate/backend/skycrateBackend/entity/RefreshToken.java

@@ -0,0 +1,33 @@
+package com.skycrate.backend.skycrateBackend.entity;
+
+import jakarta.persistence.*;
+import java.time.Instant;
+
+@Entity
+public class RefreshToken {
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    private Long id;
+
+    @Column(nullable = false, unique = true)
+    private String token;
+
+    @OneToOne
+    @JoinColumn(name = "user_id", referencedColumnName = "id")
+    private User user;
+
+    private Instant expiryDate;
+
+    // Getters and setters
+
+    public Long getId() { return id; }
+
+    public String getToken() { return token; }
+    public void setToken(String token) { this.token = token; }
+
+    public User getUser() { return user; }
+    public void setUser(User user) { this.user = user; }
+
+    public Instant getExpiryDate() { return expiryDate; }
+    public void setExpiryDate(Instant expiryDate) { this.expiryDate = expiryDate; }
+}