feat(gitea): support extra outbound HTTP headers via GITEA_EXTRA_HEADERS

Add a GITEA_EXTRA_HEADERS environment variable that accepts a JSON
object of header name/value pairs (e.g. Cloudflare Access
credentials) and applies them to every outbound request to Gitea,
both the raw pkg/gitea.DoJSON/DoBytes path and the SDK-backed
pkg/gitea.NewClient path, without overriding Authorization,
Content-Type, or Accept.

Co-Authored-By: Codet <codet@commitgo.dev> (GPT-5-Codex)

pkg/gitea/rest.go

@@ -59,7 +59,7 @@ var (
 func restHTTPClient() *http.Client {
 	restClientOnce.Do(func() {
 		restClient = &http.Client{
-			Transport:     sharedTransport(),
+			Transport:     giteaTransport(),
 			Timeout:       httpClientTimeout,
 			CheckRedirect: checkRedirect,
 		}
@@ -180,7 +180,7 @@ func DoJSON(ctx context.Context, method, path string, query url.Values, body, re
 
 func attachmentHTTPClient(origin *url.URL) *http.Client {
 	return &http.Client{
-		Transport: sharedTransport(),
+		Transport: giteaTransport(),
 		CheckRedirect: func(req *http.Request, via []*http.Request) error {
 			if err := checkRedirect(req, via); err != nil {
 				return err