test(dashboard): add unit tests for user-breakdown API

Handler tests (9 cases): group_id/model/endpoint filters, default
endpoint_type, custom limit, limit clamping, response format,
empty result, no-filter pass-through.

Repository test: resolveEndpointColumn mapping for inbound/upstream/path.

backend/internal/repository/usage_log_repo_breakdown_test.go

@@ -0,0 +1,29 @@
+//go:build unit
+
+package repository
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestResolveEndpointColumn(t *testing.T) {
+	tests := []struct {
+		endpointType string
+		want         string
+	}{
+		{"inbound", "ul.inbound_endpoint"},
+		{"upstream", "ul.upstream_endpoint"},
+		{"path", "ul.inbound_endpoint || ' -> ' || ul.upstream_endpoint"},
+		{"", "ul.inbound_endpoint"},           // default
+		{"unknown", "ul.inbound_endpoint"},     // fallback
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.endpointType, func(t *testing.T) {
+			got := resolveEndpointColumn(tc.endpointType)
+			require.Equal(t, tc.want, got)
+		})
+	}
+}