feat: forward Accept header and fix Content-Type for Docker proxying

Two fixes for Docker registry compatibility:

1. Forward the client's Accept header to upstream registries. Docker
   clients send specific Accept headers to negotiate manifest format
   (Docker v2 vs OCI). Without forwarding, registries default to OCI
   format which older Docker daemons reject.

2. Always prefer upstream's Content-Type over the provider's default.
   The provider hardcodes manifest types but upstream may return a
   different format (e.g. OCI index vs Docker manifest list).

Tested with skopeo against DockerHub, GHCR, and Quay registries.

internal/api/v1/proxy.go

@@ -67,7 +67,7 @@ func (h *ProxyHandler) handleProxy(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	result, err := h.engine.Fetch(r.Context(), *remote, path, prov)
+	result, err := h.engine.Fetch(r.Context(), *remote, path, prov, r.Header)
 	if err != nil {
 		var proxyErr *proxy.ProxyError
 		if errors.As(err, &proxyErr) {