fix(idgen): retry snowflake generation and error on negative ID

NextUint64ID now retries up to 3 times when Int64() is negative, then
returns an error instead of 0 or Fatalf. All call sites propagate the
error to avoid GORM omitting zero-value primary keys.

internal/db/idgen/snowflake_test.go

@@ -0,0 +1,17 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package idgen
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestNextUint64ID(t *testing.T) {
+	id, err := NextUint64ID()
+	require.NoError(t, err)
+	assert.NotZero(t, id)
+}