feat(config): add AppPropertiesValidator to validate required environment variables at startup

infrastructure/src/main/java/com/pablotj/restemailbridge/infrastructure/config/AppPropertiesValidator.java

@@ -0,0 +1,55 @@
+package com.pablotj.restemailbridge.infrastructure.config;
+
+import jakarta.annotation.PostConstruct;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.stereotype.Component;
+
+@Component
+public class AppPropertiesValidator {
+
+    private static final Logger log = LoggerFactory.getLogger(AppPropertiesValidator.class);
+
+    @Value("${app.encryption.secret:}")
+    private String encryptionSecret;
+
+    @Value("${app.cors.allowed-origins:}")
+    private String allowedOrigins;
+
+    @Value("${spring.datasource.url:}")
+    private String datasourceUrl;
+
+    @Value("${spring.datasource.username:}")
+    private String datasourceUser;
+
+    @Value("${spring.datasource.password:}")
+    private String datasourcePassword;
+
+    @Value("${gmail.oauth2.clientId:}")
+    private String gmailClientId;
+
+    @Value("${gmail.oauth2.clientSecret:}")
+    private String gmailClientSecret;
+
+    @PostConstruct
+    public void validate() {
+        check("APP_ENCRYPTION_SECRET", encryptionSecret, false);
+        check("APP_ALLOWED_ORIGINS", allowedOrigins, true);
+        check("DB_URL", datasourceUrl, true);
+        check("DB_USER", datasourceUser, true);
+        check("DB_PASSWORD", datasourcePassword, false);
+        check("GMAIL_OAUTH_CLIENT_ID", gmailClientId, true);
+        check("GMAIL_OAUTH_CLIENT_SECRET", gmailClientSecret, false);
+    }
+
+    private void check(String name, String value, boolean logValue) {
+        if (value == null || value.isEmpty()) {
+            throw new IllegalStateException("ERROR: Property '" + name + "' is not defined or empty");
+        } else if (logValue) {
+            log.info("Property '{}' is defined with value: {}", name, value);
+        } else {
+            log.info("Property '{}' is defined", name);
+        }
+    }
+}