docs: 统一时间格式为亚洲友好的日期时间字符串

所有文档中的时间字段统一为 yyyy-MM-dd HH:mm:ss 格式（Asia/Shanghai）：
- 移除 ISO 8601 格式（2026-02-27T10:00:00Z）
- 移除 Unix 时间戳（1740700000）
- 响应字段从 timestamp(int64) 改为 time(string)
- API README 新增「时间格式规范」章节
- 涉及 10 个文档文件

Made-with: Cursor

docs/api/im.md

@@ -49,7 +49,7 @@
                 "type": 1,
                 "content": "你好",
                 "sender_id": 2,
-                "created_at": "2026-02-27T10:30:00Z"
+                "created_at": "2026-02-27 10:30:00"
             },
             "unread_count": 3,
             "is_pinned": false
@@ -65,7 +65,7 @@
                 "type": 1,
                 "content": "明天开会",
                 "sender_id": 3,
-                "created_at": "2026-02-27T11:00:00Z"
+                "created_at": "2026-02-27 11:00:00"
             },
             "unread_count": 0,
             "is_pinned": true,
@@ -117,7 +117,7 @@
             { "user_id": 1, "nickname": "张三", "role": 2, "online": true },
             { "user_id": 2, "nickname": "李四", "role": 0, "online": false }
         ],
-        "created_at": "2026-02-20T09:00:00Z"
+        "created_at": "2026-02-20 09:00:00"
     }
 }
 ```
@@ -153,7 +153,7 @@
                 "content": "明天几点开会？",
                 "extra": {},
                 "status": 1,
-                "created_at": "2026-02-27T10:28:00Z"
+                "created_at": "2026-02-27 10:28:00"
             },
             {
                 "id": 99,
@@ -169,7 +169,7 @@
                     "thumbnail": "https://cdn.echochat.com/img/xxx_thumb.jpg"
                 },
                 "status": 1,
-                "created_at": "2026-02-27T10:29:00Z"
+                "created_at": "2026-02-27 10:29:00"
             }
         ],
         "has_more": true