feat: implement core use cases and DTOs (OAuth2, tests, and docs pending)

application/src/main/java/com/pablotj/restemailbridge/application/dto/EmailDTO.java

@@ -0,0 +1,8 @@
+package com.pablotj.restemailbridge.application.dto;
+
+public record EmailDTO(
+        String from,
+        String subject,
+        String body
+) {
+}

application/src/main/java/com/pablotj/restemailbridge/application/port/EmailConfigurationPort.java

@@ -0,0 +1,5 @@
+package com.pablotj.restemailbridge.application.port;
+
+public interface EmailConfigurationPort {
+    String getDefaultRecipient();
+}

application/src/main/java/com/pablotj/restemailbridge/application/usecase/SendEmailUseCase.java

@@ -0,0 +1,32 @@
+package com.pablotj.restemailbridge.application.usecase;
+
+import com.pablotj.restemailbridge.application.dto.EmailDTO;
+import com.pablotj.restemailbridge.application.port.EmailConfigurationPort;
+import com.pablotj.restemailbridge.domain.model.Email;
+import com.pablotj.restemailbridge.domain.repository.EmailRepository;
+import com.pablotj.restemailbridge.domain.service.EmailService;
+
+public class SendEmailUseCase {
+    private final EmailConfigurationPort emailConfigurationPort;
+    private final EmailService emailService;
+    private final EmailRepository emailRepository;
+
+    public SendEmailUseCase(EmailConfigurationPort emailConfigurationPort, EmailService emailService, EmailRepository emailRepository) {
+        this.emailConfigurationPort = emailConfigurationPort;
+        this.emailService = emailService;
+        this.emailRepository = emailRepository;
+    }
+
+    public void handle(EmailDTO emailDTO) {
+        String to = emailConfigurationPort.getDefaultRecipient();
+        Email email = emailService.sendEmail(
+                Email.builder()
+                        .from(emailDTO.from())
+                        .to(to)
+                        .subject(emailDTO.subject())
+                        .body(emailDTO.body())
+                        .build()
+        );
+        emailRepository.save(email);
+    }
+}