feat(钉钉通道): 支持仅使用 access_token 配置钉钉机器人

Motivation:
钉钉机器人的 webhook 前缀固定不变，要求用户填写完整 URL 冗余且容易出错。简化通道配置项，降低接入配置成本，同时保证存量配置不受影响。

Changes:

* 钉钉通道配置改用 access_token，webhook 前缀由内部固定拼接
* 保留对旧 webhook_url 配置的兼容，access_token 存在时优先生效
* 新增 webhook 地址解析逻辑的单元测试，覆盖优先级与回退场景
* 同步更新 README、HTTP 请求示例及设计文档中的配置示例
* E2E 测试改用 E2E_DINGTALK_ACCESS_TOKEN 环境变量并按新格式创建通道

docs/httpie/curls.md

@@ -221,7 +221,7 @@ curl -X POST 'http://localhost:8080/api/v1/channels' \
     "name": "dingtalk-prod",
     "type": "dingtalk",
     "config": {
-      "webhook_url": "https://oapi.dingtalk.com/robot/send?access_token=xxx",
+      "access_token": "xxx",
       "secret": "SEC..."
     },
     "status": 1

docs/httpie/postman_collection.json

@@ -312,7 +312,7 @@
             ],
             "body": {
               "mode": "raw",
-              "raw": "{\n  \"name\": \"dingtalk-prod\",\n  \"type\": \"dingtalk\",\n  \"config\": {\n    \"webhook_url\": \"https://oapi.dingtalk.com/robot/send?access_token=xxx\",\n    \"secret\": \"SEC...\"\n  },\n  \"status\": 1\n}"
+              "raw": "{\n  \"name\": \"dingtalk-prod\",\n  \"type\": \"dingtalk\",\n  \"config\": {\n    \"access_token\": \"xxx\",\n    \"secret\": \"SEC...\"\n  },\n  \"status\": 1\n}"
             },
             "url": "{{baseUrl}}/api/v1/channels"
           }