feat: expose rss feed endpoint

backend/src/main/java/com/openisle/controller/RssController.java

@@ -0,0 +1,78 @@
+package com.openisle.controller;
+
+import com.openisle.model.Post;
+import com.openisle.service.PostService;
+import lombok.RequiredArgsConstructor;
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.RestController;
+
+import java.time.ZoneId;
+import java.time.format.DateTimeFormatter;
+import java.util.List;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+@RestController
+@RequiredArgsConstructor
+public class RssController {
+    private final PostService postService;
+
+    @Value("${app.website-url:https://www.open-isle.com}")
+    private String websiteUrl;
+
+    private static final Pattern MD_IMAGE = Pattern.compile("!\\[[^\\]]*\\]\\(([^)]+)\\)");
+    private static final Pattern HTML_IMAGE = Pattern.compile("<img[^>]+src=[\"']?([^\"'>]+)[\"']?[^>]*>");
+
+    @GetMapping(value = "/api/rss", produces = "application/rss+xml;charset=UTF-8")
+    public String feed() {
+        List<Post> posts = postService.listLatestRssPosts(10);
+        StringBuilder sb = new StringBuilder();
+        sb.append("<?xml version=\"1.0\" encoding=\"UTF-8\"?>");
+        sb.append("<rss version=\"2.0\">");
+        sb.append("<channel>");
+        sb.append("<title>OpenIsle RSS</title>");
+        sb.append("<link>").append(websiteUrl).append("</link>");
+        sb.append("<description>Latest posts</description>");
+        DateTimeFormatter fmt = DateTimeFormatter.RFC_1123_DATE_TIME;
+        for (Post p : posts) {
+            String link = websiteUrl + "/posts/" + p.getId();
+            sb.append("<item>");
+            sb.append("<title><![CDATA[").append(p.getTitle()).append("]]></title>");
+            sb.append("<link>").append(link).append("</link>");
+            sb.append("<guid isPermaLink=\"true\">").append(link).append("</guid>");
+            sb.append("<pubDate>")
+                    .append(p.getCreatedAt().atZone(ZoneId.systemDefault()).format(fmt))
+                    .append("</pubDate>");
+            String desc = p.getContent() + "\n<p>更多细节请访问原文：" + link + "</p>";
+            sb.append("<description><![CDATA[").append(desc).append("]]></description>");
+            String img = firstImage(p.getContent());
+            if (img != null) {
+                sb.append("<enclosure url=\"").append(img).append("\" type=\"")
+                        .append(getMimeType(img)).append("\" />");
+            }
+            sb.append("</item>");
+        }
+        sb.append("</channel></rss>");
+        return sb.toString();
+    }
+
+    private String firstImage(String content) {
+        Matcher m = MD_IMAGE.matcher(content);
+        if (m.find()) {
+            return m.group(1);
+        }
+        m = HTML_IMAGE.matcher(content);
+        if (m.find()) {
+            return m.group(1);
+        }
+        return null;
+    }
+
+    private String getMimeType(String url) {
+        String lower = url.toLowerCase();
+        if (lower.endsWith(".png")) return "image/png";
+        if (lower.endsWith(".gif")) return "image/gif";
+        return "image/jpeg";
+    }
+}