refactor: generate summaries only for content changes (#7200)

#### What type of PR is this?
/kind improvement
/area core
/milestone 2.20.x

#### What this PR does / why we need it:
自动生成摘要仅对内容变更时生效

see #7193 (comment) for more details

避免对资源造成浪费如 AI 摘要生成

#### Which issue(s) this PR fixes:

Fixes #7193

#### Does this PR introduce a user-facing change?

```release-note
自动生成摘要仅对内容发生变更时生效
```

api/src/main/java/run/halo/app/core/extension/content/Constant.java

@@ -10,4 +10,6 @@ public enum Constant {
     public static final String PERMALINK_PATTERN_ANNO = "content.halo.run/permalink-pattern";
 
     public static final String CHECKSUM_CONFIG_ANNO = "checksum/config";
+
+    public static final String CONTENT_CHECKSUM_ANNO = "checksum/content";
 }

application/src/main/java/run/halo/app/core/reconciler/PostReconciler.java

@@ -57,6 +57,7 @@
 import run.halo.app.extension.ExtensionClient;
 import run.halo.app.extension.ExtensionOperator;
 import run.halo.app.extension.ListOptions;
+import run.halo.app.extension.MetadataUtil;
 import run.halo.app.extension.Ref;
 import run.halo.app.extension.controller.Controller;
 import run.halo.app.extension.controller.ControllerBuilder;
@@ -386,6 +387,16 @@ private String getExcerpt(Post post) {
             return StringUtils.EMPTY;
         }
         var content = contentWrapper.get();
+
+        var contentChecksum = Hashing.sha256().hashString(content.getContent(), UTF_8).toString();
+        var annotations = MetadataUtil.nullSafeAnnotations(post);
+        var oldChecksum = annotations.get(Constant.CONTENT_CHECKSUM_ANNO);
+        if (Objects.equals(oldChecksum, contentChecksum)) {
+            return post.getStatusOrDefault().getExcerpt();
+        }
+        // update the checksum and generate new excerpt
+        annotations.put(Constant.CONTENT_CHECKSUM_ANNO, contentChecksum);
+
         var tags = listTagDisplayNames(post);
 
         var keywords = new HashSet<>(tags);

application/src/main/java/run/halo/app/core/reconciler/SinglePageReconciler.java

@@ -3,6 +3,7 @@
 import static java.nio.charset.StandardCharsets.UTF_8;
 import static org.springframework.web.util.UriUtils.encodePath;
 
+import com.google.common.hash.Hashing;
 import java.time.Instant;
 import java.util.HashSet;
 import java.util.List;
@@ -25,6 +26,7 @@
 import run.halo.app.content.comment.CommentService;
 import run.halo.app.core.counter.CounterService;
 import run.halo.app.core.counter.MeterUtils;
+import run.halo.app.core.extension.content.Constant;
 import run.halo.app.core.extension.content.Post;
 import run.halo.app.core.extension.content.SinglePage;
 import run.halo.app.core.extension.content.Snapshot;
@@ -372,6 +374,16 @@ private String getExcerpt(SinglePage singlePage) {
             return StringUtils.EMPTY;
         }
         var content = contentWrapper.get();
+
+        var contentChecksum = Hashing.sha256().hashString(content.getContent(), UTF_8).toString();
+        var annotations = MetadataUtil.nullSafeAnnotations(singlePage);
+        var oldChecksum = annotations.get(Constant.CONTENT_CHECKSUM_ANNO);
+        if (Objects.equals(oldChecksum, contentChecksum)) {
+            return singlePage.getStatusOrDefault().getExcerpt();
+        }
+        // update the checksum and generate new excerpt
+        annotations.put(Constant.CONTENT_CHECKSUM_ANNO, contentChecksum);
+
         var context = new ExcerptGenerator.Context()
             .setRaw(content.getRaw())
             .setContent(content.getContent())