API: Add GitHub-compatible GET /repos/{owner}/{repo}/commits/{ref} endpoint

### What problem would this solve? Tools that target both GitHub and Gitea APIs need to handle two different paths for the same operation. - GitHub: `GET /repos/{owner}/{repo}/commits/{ref}` → returns a single commit - Gitea: `GET /repos/{owner}/{repo}/commits/{ref}` → 404 (only `git/commits/{sha}` works) This forces clients to implement fallback logic or detect which server they're talking to. ### What do you propose? Add a route alias at `GET /repos/{owner}/{repo}/commits/{ref}` that delegates to the existing `GetSingleCommit` handler. Currently in `routers/api/v1/api.go`: ​```go m.Group("/commits", func() { m.Get("", context.ReferencesGitRepo(), repo.GetAllCommits) // add: m.Get("/{ref}", repo.GetSingleCommit) }) ​``` This would be a one-line change that improves GitHub API compatibility.