fix(invoices): wrap refund flow in transaction for idempotency safety

- Wrap idempotency check + processRefund() + db.insert() in db.transaction()
- This prevents duplicate Stripe refunds if the DB insert fails after Stripe processes the refund
- Add migration 0027_refunds for the refunds table (was missing)
- Removes out-of-scope changes from PR #278 (csrf.ts, appointmentGroups, appointments, book, groomingLogs, services, stripe-webhooks)

Fixes GRO-637 per CTO review

Co-Authored-By: Paperclip <noreply@paperclip.ing>

packages/db/migrations/meta/_journal.json

@@ -190,6 +190,13 @@
       "when": 1775568867192,
       "tag": "0026_stripe_payment",
       "breakpoints": true
+    },
+    {
+      "idx": 27,
+      "version": "7",
+      "when": 1775655267192,
+      "tag": "0027_refunds",
+      "breakpoints": true
     }
   ]
 }