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/FormSubmissionService.php

@@ -33,6 +33,7 @@ final class FormSubmissionService
     public function __construct(
         private readonly FormLocaleResolver $localeResolver,
         private readonly FormValueService $valueService,
+        private readonly FormFieldBindingService $bindingService,
     ) {}
 
     /**
@@ -199,7 +200,7 @@ final class FormSubmissionService
      */
     private function buildSnapshot(FormSchema $schema): array
     {
-        $schema->loadMissing(['fields', 'sections']);
+        $schema->loadMissing(['fields.bindings', 'sections']);
 
         return [
             'schema_version' => $schema->version,
@@ -235,7 +236,7 @@ final class FormSubmissionService
                 'is_required' => (bool) $f->is_required,
                 'is_filterable' => (bool) $f->is_filterable,
                 'is_pii' => (bool) $f->is_pii,
-                'binding' => $f->binding,
+                'binding' => $this->bindingService->toJsonShape($f->bindings->first()),
                 'conditional_logic' => $f->conditional_logic,
                 'translations' => $f->translations,
                 'value_storage_hint' => $f->value_storage_hint instanceof \BackedEnum ? $f->value_storage_hint->value : $f->value_storage_hint,