fix(together): include invite_code in GET /together/groups/:id response

Was accidentally excluded from SELECT, causing invite code to disappear
in the settings panel.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server.js

@@ -2591,7 +2591,7 @@ ${excerpt}
     r.get('/together/groups/:groupId', async (req, res) => {
         if (!/^[a-zA-Z0-9_-]+$/.test(req.params.groupId)) return res.status(400).json({ error: 'invalid groupId' });
         try {
-            const result = await pool.query('SELECT id, name, created_at FROM together_groups WHERE id=$1', [req.params.groupId]);
+            const result = await pool.query('SELECT id, name, invite_code, created_at FROM together_groups WHERE id=$1', [req.params.groupId]);
             if (result.rows.length === 0) return res.status(404).json({ error: 'グループが見つかりません' });
             res.json(result.rows[0]);
         } catch (e) {