Split bills + kitchen ingest for online orders

Split bills (restaurant tickets):
- New pos_payments ledger; a tab is settled across one or more cash/Ladill Pay
  payments. The ticket shows total / paid / balance, an amount field with Full /
  ½ / ⅓ / ¼ helpers, and the payments taken. Partial payments keep the tab open;
  the sale finalises (and frees the table) only when the balance hits zero.
  Pay splits get their own checkout + callback (pos.payments.callback).

Pipe online orders to the KDS:
- POST /api/kitchen/orders — a first-party, service-keyed ingest
  (config pos.kitchen_api_keys, scoped by owner, idempotent by external_ref) that
  creates a paid, already-fired ticket (order_type=online, lines source=online).
- The KDS feed is now payment-agnostic (any kitchen-active sale), so paid online
  orders sit on the board next to dine-in tabs and bump the same way; they're
  badged "online".

Schema additive: pos_payments, pos_sales.external_ref. Suite green (14).

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

config/pos.php

@@ -3,4 +3,10 @@
 return [
     'default_currency' => env('POS_DEFAULT_CURRENCY', 'GHS'),
     'merchant_import_enabled' => (bool) env('POS_MERCHANT_IMPORT_ENABLED', true),
+
+    // Service-to-service keys for the kitchen ingest API (POST /api/kitchen/orders),
+    // keyed by the calling service. Empty = that service cannot push (secure default).
+    'kitchen_api_keys' => array_filter([
+        'merchant' => env('KITCHEN_API_KEY_MERCHANT'),
+    ]),
 ];