feat(form-builder): FormFieldBindingService + library-to-field row copy + snapshot writer

WS-5a commit 2 of 4.

FormFieldBindingService owns all writes to the relational binding table.
Validation against config/form_binding.php entity-column registry lives here
(ARCH §6.2).

FormFieldService::insertFromLibrary now calls copyBindings instead of
hydrating JSON — the Q3 row-copy mandate. Library and field bindings share
the same table; insertion is a row-clone operation.

Snapshot writer (FormSubmissionService::buildSnapshot) serialises bindings
via toJsonShape so schema_snapshot JSON keeps its ARCH §4.6.1 / §6.3
contract. No snapshot format change.
API resources source binding output from the relational table via the same
serialiser — external shape preserved.

Tests: service transactional behaviour, copyBindings preservation,
snapshot parity, API resource parity.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

api/app/Services/FormBuilder/FormFieldService.php

@@ -26,6 +26,7 @@ final class FormFieldService
 {
     public function __construct(
         private readonly FormSchemaService $schemaService,
+        private readonly FormFieldBindingService $bindingService,
     ) {}
 
     public function create(FormSchema $schema, array $data): FormField
@@ -140,7 +141,6 @@ final class FormFieldService
             'validation_rules' => $library->validation_rules,
             'is_required' => (bool) $library->default_is_required,
             'is_filterable' => (bool) $library->default_is_filterable,
-            'binding' => $library->default_binding,
             'translations' => $library->translations,
             'sort_order' => $this->nextSortOrder($schema),
         ], $overrides);
@@ -154,6 +154,8 @@ final class FormFieldService
         /** @var FormField $field */
         $field = FormField::create($data);
 
+        $this->bindingService->copyBindings($library, $field);
+
         FormFieldLibrary::query()->whereKey($library->id)->increment('usage_count');
 
         $this->schemaService->bumpVersion($schema);