Restaurant mode Phase 3: table-QR self-ordering into the kitchen

The "links" slice — a guest scans a table's QR, browses the menu (with
modifiers), and submits an order that lands on that table's open tab and fires
straight to the Kitchen Display.

- Public, auth-free flow scoped by an unguessable table short_code:
  GET /t/{code} (menu + client cart), POST /t/{code}/order (throttled),
  GET /t/{code}/done. Orders open/append the table's dine-in tab, add lines as
  source=guest, and send to the kitchen.
- Staff print a per-table QR (Settings → table → QR; renders client-side to the
  public menu URL). short_code is generated lazily.
- Guest lines are badged on the ticket and the KDS so staff can tell them apart;
  staff still settle the tab as usual (cash / Ladill Pay).
- Extracted PosSaleService::buildProductLine as the single product+modifier
  price resolver, now shared by staff and guest ordering (client prices never
  trusted).

Schema additive: pos_tables.short_code, pos_sale_lines.source. New
PosRestaurantTest covers the guest order firing to the kitchen; suite green (12).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/Models/PosSaleLine.php

@@ -38,6 +38,7 @@ class PosSaleLine extends Model
         'line_total_minor',
         'position',
         'kitchen_state',
+        'source',
         'notes',
         'course',
     ];