feat: forward Accept header and fix Content-Type for Docker proxying (#62)

## Problems
1. Docker daemon sends specific Accept headers to negotiate manifest format, but the proxy dropped them — registries defaulted to OCI format, causing "mediaType should be manifest.v2+json not oci.image.index" errors
2. Upstream Content-Type was only used when the provider returned "application/octet-stream" — Docker manifests got the wrong Content-Type

## Fixes
- Forward client Accept header to upstream (both initial request and Bearer token retry)
- Always prefer upstream Content-Type when present
- Fetch signature now accepts variadic clientHeaders for backwards compat

## E2E tested
- DockerHub: redis:7-alpine, alpine:3 — skopeo inspect OK
- GHCR: OCI-only images work with docker pull (GHCR 404s Docker v2 Accept, which is expected)
- Quay: prometheus/node-exporter — skopeo inspect OK

Reviewed-on: #62
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

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) {