perf: 优化快照接口——缓存预序列化 JSON，消除重复序列化

- Caffeine 缓存改为存储 bundle JSON 字符串而非 SiteSnapshot 对象（减少内存占用）
- 新增 getActiveBundleJson 方法，SnapshotController 直接使用缓存 JSON
- bundle JSON 新增 checksum 字段，ETag 比较无需额外查库
- 删除 SnapshotController.toJson 重复序列化代码
- publish 时复用已序列化的 JSON 上传 CDN

par-api/src/main/java/com/par/api/controller/SnapshotController.java

@@ -1,5 +1,6 @@
 package com.par.api.controller;
 
+import com.fasterxml.jackson.databind.JsonNode;
 import com.fasterxml.jackson.databind.ObjectMapper;
 import com.par.core.dto.ApiResponse;
 import com.par.core.entity.SiteSnapshot;
@@ -32,7 +33,7 @@ public class SnapshotController {
     /**
      * 获取站点当前生效配置快照（供 mediabot 使用）
      * 支持 ETag (If-None-Match) 实现增量更新
-     * 优先从数据库读取，数据库不可用时回退到七牛云 CDN
+     * 优先从 Caffeine 缓存读取，缓存未命中查数据库并回退七牛云 CDN
      */
     @GetMapping("/{siteId}")
     public ResponseEntity<?> getSnapshot(
@@ -40,9 +41,9 @@ public class SnapshotController {
             @RequestHeader(value = "If-None-Match", required = false) String ifNoneMatch) {
 
         try {
-            SiteSnapshot active = snapshotService.getActive(siteId);
+            String bundleJson = snapshotService.getActiveBundleJson(siteId);
 
-            if (active == null) {
+            if (bundleJson == null || bundleJson.isEmpty()) {
                 // 尝试从云存储 CDN 回退
                 String cached = storageService.read("snapshots/" + siteId + ".json");
                 if (cached != null && !cached.isBlank()) {
@@ -53,18 +54,18 @@ public class SnapshotController {
                 return ResponseEntity.notFound().build();
             }
 
-            // ETag 检查
-            if (ifNoneMatch != null && ifNoneMatch.equals(active.getChecksum())) {
+            // ETag 检查：从 JSON 中提取 checksum
+            String etag = extractChecksum(bundleJson);
+            if (ifNoneMatch != null && etag != null && ifNoneMatch.equals(etag)) {
                 return ResponseEntity.status(304)
-                        .header(HttpHeaders.ETAG, active.getChecksum())
+                        .header(HttpHeaders.ETAG, etag)
                         .build();
             }
 
-            String json = toJson(active);
             return ResponseEntity.ok()
-                    .header(HttpHeaders.ETAG, active.getChecksum())
+                    .header(HttpHeaders.ETAG, etag)
                     .contentType(MediaType.APPLICATION_JSON)
-                    .body(json);
+                    .body(bundleJson);
 
         } catch (Exception e) {
             log.error("获取快照失败 siteId={}", siteId, e);
@@ -99,39 +100,13 @@ public class SnapshotController {
         return ApiResponse.success(result);
     }
 
-    private String toJson(SiteSnapshot snapshot) {
+    /** 从 bundle JSON 中提取 checksum 字段，用于 ETag 比较 */
+    private String extractChecksum(String bundleJson) {
         try {
-            Map<String, Object> bundle = new LinkedHashMap<>();
-            bundle.put("siteId", snapshot.getSiteId());
-            bundle.put("generatedAt", snapshot.getPublishedAt() != null
-                    ? snapshot.getPublishedAt().toString()
-                    : java.time.LocalDateTime.now().toString());
-
-            if (snapshot.getScript() != null) {
-                bundle.put("script", snapshot.getScript());
-                bundle.put("scriptVersion", snapshot.getScriptVersion());
-                bundle.put("scriptSource", snapshot.getScriptSource());
-            }
-
-            if (snapshot.getPages() != null) {
-                @SuppressWarnings("unchecked")
-                var pages = om.readValue(snapshot.getPages(), java.util.List.class);
-                bundle.put("pages", pages);
-                bundle.put("pagesVersion", snapshot.getPagesVersion());
-                bundle.put("pagesSource", snapshot.getPagesSource());
-            }
-
-            if (snapshot.getRules() != null) {
-                @SuppressWarnings("unchecked")
-                var rules = om.readValue(snapshot.getRules(), java.util.List.class);
-                bundle.put("riskRules", rules);
-                bundle.put("riskRulesVersion", snapshot.getRulesVersion());
-            }
-
-            return om.writeValueAsString(bundle);
+            JsonNode node = om.readTree(bundleJson);
+            return node.has("checksum") ? node.get("checksum").asText() : null;
         } catch (Exception e) {
-            log.error("序列化快照失败 siteId={}", snapshot.getSiteId(), e);
-            return "{}";
+            return null;
         }
     }
 }

par-core/src/main/java/com/par/core/service/SiteSnapshotService.java

@@ -34,6 +34,13 @@ public interface SiteSnapshotService {
      */
     SiteSnapshot getActive(String siteId);
 
+    /**
+     * 获取站点当前生效快照的预序列化 JSON（带 Caffeine 缓存）
+     * 供 SnapshotController 使用，避免重复序列化
+     * @return JSON 字符串，或 null 表示无快照
+     */
+    String getActiveBundleJson(String siteId);
+
     /**
      * 按 checksum 判断是否需要更新
      */

par-core/src/main/java/com/par/core/service/impl/SiteSnapshotServiceImpl.java

@@ -43,8 +43,8 @@ public class SiteSnapshotServiceImpl implements SiteSnapshotService {
     private final CachedStorageService storageService;
     private static final ObjectMapper om = new ObjectMapper();
 
-    /** 已发布快照本地缓存：key=siteId, value=SiteSnapshot */
-    private final Cache<String, SiteSnapshot> activeSnapshotCache = Caffeine.newBuilder()
+    /** 已发布快照 JSON 缓存：key=siteId, value=预序列化的 bundle JSON */
+    private final Cache<String, String> bundleJsonCache = Caffeine.newBuilder()
             .expireAfterWrite(Duration.ofMinutes(10))
             .maximumSize(500)
             .build();
@@ -193,12 +193,12 @@ public class SiteSnapshotServiceImpl implements SiteSnapshotService {
         draft.setPublishedAt(LocalDateTime.now());
         snapshotMapper.updateById(draft);
 
-        // 刷新本地缓存
-        activeSnapshotCache.put(siteId, draft);
+        // 序列化并刷新缓存
+        String bundleJson = toBundleJson(draft);
+        bundleJsonCache.put(siteId, bundleJson);
 
-        // 上传到七牛云 CDN 作为灾备
+        // 上传到七牛云 CDN 作为灾备（复用已序列化的 JSON）
         try {
-            String bundleJson = toBundleJson(draft);
             String cdnPath = "snapshots/" + siteId + ".json";
             storageService.store(cdnPath, bundleJson);
             // 也保存版本化副本
@@ -217,9 +217,19 @@ public class SiteSnapshotServiceImpl implements SiteSnapshotService {
 
     @Override
     public SiteSnapshot getActive(String siteId) {
-        return activeSnapshotCache.get(siteId, key -> {
-            log.debug("缓存未命中，查询数据库 siteId={}", siteId);
-            return snapshotMapper.selectLatestPublished(siteId);
+        return snapshotMapper.selectLatestPublished(siteId);
+    }
+
+    @Override
+    public String getActiveBundleJson(String siteId) {
+        return bundleJsonCache.get(siteId, key -> {
+            SiteSnapshot active = snapshotMapper.selectLatestPublished(key);
+            if (active == null) {
+                log.debug("站点 {} 无 published 快照", key);
+                return "";  // 空字符串表示无数据，Caffeine 会缓存
+            }
+            log.debug("缓存未命中，序列化 siteId={}", key);
+            return toBundleJson(active);
         });
     }
 
@@ -253,6 +263,7 @@ public class SiteSnapshotServiceImpl implements SiteSnapshotService {
         try {
             Map<String, Object> bundle = new LinkedHashMap<>();
             bundle.put("siteId", snapshot.getSiteId());
+            bundle.put("checksum", snapshot.getChecksum());
             bundle.put("generatedAt", snapshot.getPublishedAt() != null
                     ? snapshot.getPublishedAt().toString() : LocalDateTime.now().toString());