feat(t14-v1234): 消息迁移增强——C2C全量分页+群消息迁移+429限速重试+回溯天数可配

V1: migrateMessages 7天窗滚动 + Complete分页拉全量历史
V2: migrateGroupMessages + getGroupMsgAs/importGroupMsg + /migrate/groupmsg
V3: importMsg/importGroupMsg 捕获429重试 + app.yml imutil.migrate(msgLookbackDays/importMaxRetry/importRetryMs)

Co-Authored-By: Claude <noreply@anthropic.com>

src/main/java/com/imutil/controller/AdminController.java

@@ -373,6 +373,33 @@ public class AdminController {
         ctx.redirect(basePath + "/admin/migrate?msg=gdone&migrated=" + task.getProcessedCount());
     }
 
+    /**
+     * 群消息迁移：上传/粘贴群 ID 清单 → 逐群拉历史消息 → 加前缀 → importGroupMsg
+     */
+    @Post
+    @Mapping("/migrate/groupmsg")
+    public void migrateGroupMsg(@Param Long sourceAppId,
+                                @Param(defaultValue = "") String userText,
+                                Context ctx) throws Throwable {
+        UploadedFile file = ctx.file("file");
+        List<String> groupIds = new ArrayList<>();
+        if (file != null) {
+            parseUserIds(new String(file.getContentAsBytes(), StandardCharsets.UTF_8), groupIds);
+        } else if (!userText.isEmpty()) {
+            parseUserIds(userText, groupIds);
+        }
+        if (sourceAppId == null) {
+            ctx.redirect(basePath + "/admin/migrate?msg=noapp");
+            return;
+        }
+        if (groupIds.isEmpty()) {
+            ctx.redirect(basePath + "/admin/migrate?msg=empty");
+            return;
+        }
+        MigrateTask task = migrateService.migrateGroupMessages(sourceAppId, groupIds);
+        ctx.redirect(basePath + "/admin/migrate?msg=gmdone&imported=" + task.getProcessedCount());
+    }
+
     /**
      * 单聊消息迁移：上传会话清单（每行 from,to）或粘贴 → getRoamMsg 拉 → 加前缀 → importMsg
      */