Add pre/post interceptor hooks to grpcserver.New

New Options parameter with PreInterceptors and PostInterceptors
allows services to inject custom interceptors into the chain:

  [pre-interceptors] → logging → auth → [post-interceptors] → handler

This enables services like metacrypt to add seal-check (pre-auth)
and audit-logging (post-auth) interceptors while using the shared
auth and logging infrastructure.

Pass nil for the default chain (logging + auth only).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

grpcserver/server_test.go

@@ -216,7 +216,7 @@ func TestNewWithoutTLS(t *testing.T) {
 	defer srv.Close()
 	a := testAuth(t, srv.URL)
 
-	s, err := New("", "", a, testMethods, slog.Default())
+	s, err := New("", "", a, testMethods, slog.Default(), nil)
 	if err != nil {
 		t.Fatalf("New: %v", err)
 	}