refactor(idgen): centralize negative ID handling via panic

Restore NextUint64ID() to uint64-only API so callers need no error
checks. Retry logic stays in idgen; after 3 negative values it panics
instead of returning 0, preventing silent NULL primary keys.

internal/db/idgen/snowflake_test.go

@@ -7,11 +7,9 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/assert"
-	"github.com/stretchr/testify/require"
 )
 
 func TestNextUint64ID(t *testing.T) {
-	id, err := NextUint64ID()
-	require.NoError(t, err)
+	id := NextUint64ID()
 	assert.NotZero(t, id)
 }