Public-only repository tokens can update private PR head branches

Affected: Go: code.gitea.io/gitea

### Summary Gitea allows a `public-only,write:repository` token to update a private pull request head branch through a public base repository route.

The vulnerable endpoint is:

```text POST /api/v1/repos/{public-owner}/{public-repo}/pulls/{index}/update ```

Gitea checks the token's public-only restriction against the route repository, which is the public base repository. `UpdatePullRequest()` then authorizes the pull request head repository with ordinary user RBAC and calls the pull update service. If the head repository is private, the active token's public-only restriction is not re-applied to that private repository before Gitea pushes changes into it.

As a result, the same token that cannot directly write to the private repository can still cause Gitea to push public base commits into the private head branch.

### Details The pull request API routes are attached under a repository route group. The public-only check applies to `ctx.Repo.Repository`, the route/base repository.

```go // routers/api/v1/api.go:1358-1394 m.Group("/pulls", func() { m.Combo("").Get(repo.ListPullRequests). Post(reqToken(), mustNotBeArchived, bind(api.CreatePullRequestOption{}), repo.CreatePullRequest) m.Get("/pinned", repo.ListPinnedPullRequests) m.Post("/comments/{id}/resolve", reqToken(), mustNotBeArchived, repo.ResolvePullReviewComment) m.Post("/comments/{id}/unresolve", reqToken(), mustNotBeArchived, repo.UnresolvePullReviewComment) m.Group("/{in…