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/Http/Resources/FormBuilder/FormFieldResource.php

@@ -7,6 +7,7 @@ namespace App\Http\Resources\FormBuilder;
 use App\Enums\FormBuilder\FormFieldType;
 use App\Models\FormBuilder\FormField;
 use App\Models\PersonTag;
+use App\Services\FormBuilder\FormFieldBindingService;
 use App\Services\FormBuilder\FormLocaleResolver;
 use Illuminate\Http\Request;
 use Illuminate\Http\Resources\Json\JsonResource;
@@ -49,7 +50,9 @@ final class FormFieldResource extends JsonResource
             'is_unique' => (bool) $this->is_unique,
             'is_pii' => (bool) $this->is_pii,
             'display_width' => $this->display_width instanceof \BackedEnum ? $this->display_width->value : $this->display_width,
-            'binding' => $this->binding,
+            'binding' => app(FormFieldBindingService::class)->toJsonShape(
+                $this->resource->bindings->first(),
+            ),
             'conditional_logic' => $this->conditional_logic,
             'role_restrictions' => $this->role_restrictions,
             'translations' => $this->translations,