feat(form-builder): form_field_bindings table + polymorphic owner + cascade observer

WS-5a commit 1 of 4 per ARCH-CONSOLIDATION-ADDENDUM-2026-04-24 Q3.

Creates the relational home for what was form_fields.binding JSON and
form_field_library.default_binding JSON. Owner discriminator is polymorphic
morph (owner_type/owner_id) — the pattern the rest of WS-5 (5b validation_rules,
5d options) will reuse.

Migration backfills rows from both JSON sources in a single transaction and
is genuinely reversible (rollback reconstructs the JSON). Old columns remain
in place until commit 3 has switched all readers.

Pattern B (binding=null) is represented by absence of row. mode enum covers
entity_owned / mirrored only.

Cascade on owner delete via observer — bindings are physical state, not
historical audit. FormFieldBindingScope enforces multi-tenancy via UNION over
both owner chains (form_field → schema → org OR form_field_library → org) —
Q2's declarative tenantScopeStrategy() can't walk morph parents.

Tests: migration forward/back, morph relation, cascade observer, scope
isolation, enum coverage.

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

api/database/factories/FormBuilder/FormFieldLibraryFactory.php

@@ -4,7 +4,9 @@ declare(strict_types=1);
 
 namespace Database\Factories\FormBuilder;
 
+use App\Enums\FormBuilder\FormFieldBindingMode;
 use App\Enums\FormBuilder\FormFieldType;
+use App\Models\FormBuilder\FormFieldBinding;
 use App\Models\FormBuilder\FormFieldLibrary;
 use App\Models\Organisation;
 use Illuminate\Database\Eloquent\Factories\Factory;
@@ -45,4 +47,29 @@ final class FormFieldLibraryFactory extends Factory
     {
         return $this->state(fn () => ['is_system' => true]);
     }
+
+    /**
+     * Attach a binding row in `form_field_bindings` after the library entry
+     * is persisted. Replaces the legacy `default_binding` JSON column.
+     */
+    public function withDefaultBinding(
+        string $entity,
+        string $attribute,
+        FormFieldBindingMode $mode = FormFieldBindingMode::EntityOwned,
+        ?string $syncDirection = null,
+    ): static {
+        return $this->afterCreating(function (FormFieldLibrary $library) use ($entity, $attribute, $mode, $syncDirection): void {
+            FormFieldBinding::factory()
+                ->forLibrary($library)
+                ->state([
+                    'target_entity' => $entity,
+                    'target_attribute' => $attribute,
+                    'mode' => $mode->value,
+                    'sync_direction' => $mode === FormFieldBindingMode::Mirrored
+                        ? ($syncDirection ?? 'write_on_submit')
+                        : null,
+                ])
+                ->create();
+        });
+    }
 }