Introduce pluggable email and image upload

src/main/java/com/openisle/service/ResendEmailSender.java

@@ -0,0 +1,39 @@
+package com.openisle.service;
+
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.http.HttpEntity;
+import org.springframework.http.HttpHeaders;
+import org.springframework.http.HttpMethod;
+import org.springframework.http.MediaType;
+import org.springframework.stereotype.Service;
+import org.springframework.web.client.RestTemplate;
+
+import java.util.HashMap;
+import java.util.Map;
+
+@Service
+public class ResendEmailSender extends EmailSender {
+
+    @Value("${resend.api.key}")
+    private String apiKey;
+
+    private final RestTemplate restTemplate = new RestTemplate();
+
+    @Override
+    public void sendEmail(String to, String subject, String text) {
+        String url = "https://api.resend.com/emails"; // hypothetical endpoint
+
+        HttpHeaders headers = new HttpHeaders();
+        headers.setContentType(MediaType.APPLICATION_JSON);
+        headers.set("Authorization", "Bearer " + apiKey);
+
+        Map<String, String> body = new HashMap<>();
+        body.put("to", to);
+        body.put("subject", subject);
+        body.put("text", text);
+        body.put("from", "openisle <noreply@chenjiating.com>"); // todo(tim): use config
+
+        HttpEntity<Map<String, String>> entity = new HttpEntity<>(body, headers);
+        restTemplate.exchange(url, HttpMethod.POST, entity, String.class);
+    }
+}