añadidos los ficheros nuevos2

src/main/java/com/imprimelibros/erp/externalApi/AuthService.java

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+package com.imprimelibros.erp.externalApi;
+
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.http.*;
+import org.springframework.stereotype.Service;
+import org.springframework.web.client.RestTemplate;
+
+import java.util.Map;
+
+@Service
+public class AuthService {
+
+    @Value("${safekat.api.url}")
+    private String skApiUrl;
+    @Value("${safekat.api.email}")
+    private String skApiEmail;
+    @Value("${safekat.api.password}")
+    private String skApiPassword;
+
+    private final RestTemplate restTemplate = new RestTemplate();
+    private String currentToken;
+
+    public synchronized String getToken() {
+        if (currentToken == null) {
+            currentToken = fetchNewToken();
+        }
+        return currentToken;
+    }
+
+    public synchronized void invalidateToken() {
+        currentToken = null;
+    }
+
+    private String fetchNewToken() {
+        Map<String, String> credentials = Map.of(
+                "email", this.skApiEmail,
+                "password", this.skApiPassword);
+        ResponseEntity<Map> response = restTemplate.postForEntity(this.skApiUrl + "auth/jwt", credentials, Map.class);
+        return (String) response.getBody().get("access_token");
+    }
+}