Fix AI review reliability and diff coverage

docs/design.md

@@ -99,11 +99,14 @@ The guard is process-local. Multi-instance deployments need a shared lock in SQL
 Required GitLab REST API calls:
 
 ```text
-GET /projects/:id/merge_requests/:merge_request_iid/changes
+GET /projects/:id/merge_requests/:merge_request_iid
+GET /projects/:id/merge_requests/:merge_request_iid/diffs?page=:page&per_page=100
 GET /projects/:id/repository/archive.zip
 POST /projects/:id/merge_requests/:merge_request_iid/discussions
 ```
 
+Diff pages are fetched until pagination is complete. File-level `collapsed`, `too_large`, and `generated_file` metadata is preserved; unavailable diffs make review coverage incomplete. The deprecated `/changes` endpoint is used only as a compatibility fallback when `/diffs` is unavailable, and its `overflow` flag is also treated as incomplete coverage.
+
 The service uses `[gitlab].token`, not the webhook secret, for API calls. The token needs `api` scope and permission to read MR diffs, download repository archives, and publish MR discussions.
 
 ## Diff Parser

docs/gitlab-webhook.md

@@ -27,11 +27,14 @@ Webhook 页面里的 `Secret token` 只用于校验收到的请求是否来自 G
 当前服务会用这个 token 调用：
 
 ```text
-GET /projects/:id/merge_requests/:merge_request_iid/changes
+GET /projects/:id/merge_requests/:merge_request_iid
+GET /projects/:id/merge_requests/:merge_request_iid/diffs?page=:page&per_page=100
 GET /projects/:id/repository/archive.zip
 POST /projects/:id/merge_requests/:merge_request_iid/discussions
 ```
 
+服务会完整读取 `/diffs` 的分页，并记录 `collapsed`、`too_large`、`generated_file` 状态。无法取得完整 diff 时，Dashboard 会显示不完整覆盖率。只有旧 GitLab 不支持 `/diffs` 时才兼容回退到已弃用的 `/changes`，并检查其 `overflow` 标志。
+
 因此 token 需要能读取 MR diff、读取仓库 archive，并发布 MR discussion。
 
 ## Merge request events 什么时候触发

src/dashboard/views.rs

@@ -224,7 +224,12 @@ pub const DASHBOARD_HTML: &str = r##"<!doctype html>
     const coverageReason = (reason) => ({
       max_batches_reached: "达到批次上限",
       single_file_diff_truncated: "单文件 Diff 超过批次限制",
-      batch_execution_failed: "批次执行失败"
+      single_diff_line_too_large: "单行 Diff 超过批次限制",
+      batch_execution_failed: "批次执行失败",
+      gitlab_diff_unavailable: "GitLab 未提供 Diff 内容",
+      gitlab_diff_collapsed: "GitLab 已折叠 Diff",
+      gitlab_diff_too_large: "Diff 超过 GitLab 大小限制",
+      gitlab_diff_overflow: "Merge Request Diff 超过 GitLab 限制"
     }[reason] || reason || "-");
     const errorCodeText = (code) => ({
       gitlab_api_timeout: "GitLab API 超时", gitlab_api_failed: "GitLab API 失败",