feat: handle Docker Bearer token auth for upstream registries

When an upstream registry returns 401 with a Www-Authenticate: Bearer
challenge, the proxy now fetches an anonymous (or authenticated) token
from the auth endpoint and retries the request.

This fixes Docker Hub pulls which require token exchange even for
public images.

internal/proxy/engine.go

@@ -4,10 +4,12 @@ import (
 	"context"
 	"crypto/sha256"
 	"encoding/hex"
+	"encoding/json"
 	"fmt"
 	"io"
 	"log/slog"
 	"net/http"
+	"strings"
 	"time"
 
 	"git.unkin.net/unkin/artifactapi/internal/cache"
@@ -147,6 +149,21 @@ func (e *Engine) fetchFromUpstream(ctx context.Context, remote models.Remote, pa
 		return nil, &UpstreamError{Err: err}
 	}
 
+	if resp.StatusCode == http.StatusUnauthorized {
+		resp.Body.Close()
+		token, err := fetchBearerToken(ctx, resp.Header.Get("Www-Authenticate"), remote)
+		if err == nil && token != "" {
+			req2, _ := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+			req2.Header.Set("Authorization", "Bearer "+token)
+			resp, err = http.DefaultClient.Do(req2)
+			if err != nil {
+				return nil, &UpstreamError{Err: err}
+			}
+		} else {
+			return nil, &ProxyError{Status: http.StatusUnauthorized, Message: "upstream returned 401"}
+		}
+	}
+
 	if resp.StatusCode != http.StatusOK {
 		resp.Body.Close()
 		return nil, &ProxyError{Status: resp.StatusCode, Message: fmt.Sprintf("upstream returned %d", resp.StatusCode)}
@@ -319,6 +336,71 @@ func (r readerAt) ReadAt(p []byte, off int64) (n int, err error) {
 	return
 }
 
+func fetchBearerToken(ctx context.Context, wwwAuth string, remote models.Remote) (string, error) {
+	if !strings.HasPrefix(wwwAuth, "Bearer ") {
+		return "", fmt.Errorf("not a Bearer challenge")
+	}
+
+	params := map[string]string{}
+	for _, part := range strings.Split(wwwAuth[7:], ",") {
+		part = strings.TrimSpace(part)
+		eq := strings.Index(part, "=")
+		if eq < 0 {
+			continue
+		}
+		key := part[:eq]
+		val := strings.Trim(part[eq+1:], `"`)
+		params[key] = val
+	}
+
+	realm := params["realm"]
+	if realm == "" {
+		return "", fmt.Errorf("no realm in Bearer challenge")
+	}
+
+	tokenURL := realm
+	sep := "?"
+	if s, ok := params["service"]; ok {
+		tokenURL += sep + "service=" + s
+		sep = "&"
+	}
+	if s, ok := params["scope"]; ok {
+		tokenURL += sep + "scope=" + s
+	}
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, tokenURL, nil)
+	if err != nil {
+		return "", err
+	}
+
+	if remote.Username != "" && remote.Password != "" {
+		req.SetBasicAuth(remote.Username, remote.Password)
+	}
+
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return "", err
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return "", fmt.Errorf("token endpoint returned %d", resp.StatusCode)
+	}
+
+	var tokenResp struct {
+		Token       string `json:"token"`
+		AccessToken string `json:"access_token"`
+	}
+	if err := json.NewDecoder(resp.Body).Decode(&tokenResp); err != nil {
+		return "", err
+	}
+
+	if tokenResp.Token != "" {
+		return tokenResp.Token, nil
+	}
+	return tokenResp.AccessToken, nil
+}
+
 type ProxyError struct {
 	Status  int
 	Message string