Fix null metadata crash on invoice_items insert

JSONB column has NOT NULL constraint — pass empty array instead of null
when no price override metadata exists.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/Livewire/Receptionist/NewRegistrationWizard.php

@@ -909,7 +909,7 @@ public function confirm(): void
                             'unit_price' => $programFee,
                             'discount_amount' => 0,
                             'tax_amount' => 0,
-                            'metadata' => $programItemMeta ?: null,
+                            'metadata' => $programItemMeta ?: [],
                         ];
                     }