fix: include upstream error details in usage API error response

When FetchUsageWithOptions receives a non-200 response from the
Anthropic API (e.g. 429 Rate Limited, 401 Unauthorized), the error
was wrapped with fmt.Errorf which infraerrors.FromError cannot
recognize, causing a generic "internal error" message with no details.

Replace fmt.Errorf with infraerrors.New(500, "UPSTREAM_ERROR", msg)
so the upstream error details (status code + body) are included in
the 500 response message. The HTTP status remains 500 to avoid
interfering with frontend auth routing (e.g. 401 would trigger
JWT expiry redirect).

backend/internal/repository/claude_usage_service.go

@@ -8,6 +8,7 @@ import (
 	"net/http"
 	"time"
 
+	infraerrors "github.com/Wei-Shaw/sub2api/internal/pkg/errors"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/httpclient"
 	"github.com/Wei-Shaw/sub2api/internal/service"
 )
@@ -95,7 +96,8 @@ func (s *claudeUsageService) FetchUsageWithOptions(ctx context.Context, opts *se
 
 	if resp.StatusCode != http.StatusOK {
 		body, _ := io.ReadAll(resp.Body)
-		return nil, fmt.Errorf("API returned status %d: %s", resp.StatusCode, string(body))
+		msg := fmt.Sprintf("API returned status %d: %s", resp.StatusCode, string(body))
+		return nil, infraerrors.New(http.StatusInternalServerError, "UPSTREAM_ERROR", msg)
 	}
 
 	var usageResp service.ClaudeUsageResponse