refactor: 合并三级缓存为统一缓存 + 重命名 saveDraft → savePreview

1. 三个 Caffeine Cache 合并为一个 jsonCache，key 用 :published/:draft 后缀区分
2. saveDraft → savePreview，更准确反映「预览版」语义（非临时草稿）
3. getPreviewBundleJson 去掉独立缓存，直接查 DB（preview 是 draft+published 逻辑并集，缓存意义不大）
4. 前端按钮「保存草稿」→「保存预览版」，API 路径 /save-draft → /save-preview

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

@@ -380,22 +380,22 @@ public class AdminController {
         } catch (Exception e) { return ApiResponse.error(500, "发布失败: " + e.getMessage()); }
     }
 
-    /** 保存站点配置草稿（预览版，不上传 CDN） */
-    @PostMapping("/sites/{siteId}/save-draft")
-    public ApiResponse<Map<String, Object>> saveDraft(@PathVariable("siteId") String siteId,
-                                                       HttpServletRequest request) {
+    /** 保存站点配置预览版（不上传 CDN） */
+    @PostMapping("/sites/{siteId}/save-preview")
+    public ApiResponse<Map<String, Object>> savePreview(@PathVariable("siteId") String siteId,
+                                                         HttpServletRequest request) {
         checkAdmin(request);
         try {
-            var draft = snapshotService.saveDraft(siteId);
-            if (draft == null) {
-                return ApiResponse.error(400, "该站点无有效配置，无法保存草稿");
+            var preview = snapshotService.savePreview(siteId);
+            if (preview == null) {
+                return ApiResponse.error(400, "该站点无有效配置，无法保存预览版");
             }
             return ApiResponse.success(Map.of(
-                    "siteId", draft.getSiteId(),
-                    "version", draft.getVersion(),
-                    "checksum", draft.getChecksum().substring(0, 8)));
+                    "siteId", preview.getSiteId(),
+                    "version", preview.getVersion(),
+                    "checksum", preview.getChecksum().substring(0, 8)));
         } catch (Exception e) {
-            return ApiResponse.error(500, "保存草稿失败: " + e.getMessage());
+            return ApiResponse.error(500, "保存预览版失败: " + e.getMessage());
         }
     }
 
@@ -407,7 +407,7 @@ public class AdminController {
         try {
             var published = snapshotService.publish(siteId);
             if (published == null) {
-                return ApiResponse.error(400, "该站点无 draft 快照可发布");
+                return ApiResponse.error(400, "该站点无预览版快照可发布");
             }
             return ApiResponse.success(Map.of(
                     "siteId", published.getSiteId(),