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/app/Models/Scopes/FormFieldBindingScope.php

@@ -0,0 +1,106 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Models\Scopes;
+
+use App\Models\FormBuilder\FormField;
+use App\Models\FormBuilder\FormFieldLibrary;
+use App\Models\FormBuilder\FormSchema;
+use Illuminate\Database\Eloquent\Builder;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Scope;
+
+/**
+ * Multi-tenant isolation for `form_field_bindings`. The table has a
+ * polymorphic owner that points at either `form_field` or
+ * `form_field_library`; `OrganisationScope` (Q2 FK-chain resolver) can't
+ * walk a morph parent, so this scope does the equivalent UNION walk:
+ *
+ *   owner_id ∈ (
+ *     SELECT id FROM form_fields
+ *       WHERE form_schema_id ∈ (SELECT id FROM form_schemas WHERE organisation_id = ?)
+ *     UNION
+ *     SELECT id FROM form_field_library
+ *       WHERE organisation_id = ?
+ *   )
+ *
+ * Organisation context resolution mirrors `OrganisationScope` — explicit
+ * override via constructor, then the `organisation` / `event` route
+ * parameter fallbacks. CLI, queues, and unauthenticated flows skip the
+ * scope (consistent with OrganisationScope).
+ *
+ * Escape hatch: callers that need cross-tenant reads use
+ * `FormFieldBinding::withoutGlobalScope(FormFieldBindingScope::class)`.
+ */
+final class FormFieldBindingScope implements Scope
+{
+    public function __construct(
+        private readonly ?string $organisationId = null,
+    ) {}
+
+    public function apply(Builder $builder, Model $model): void
+    {
+        $orgId = $this->resolveOrganisationId();
+        if ($orgId === null) {
+            return;
+        }
+
+        $fieldIds = FormField::query()
+            ->withoutGlobalScope(OrganisationScope::class)
+            ->whereIn(
+                'form_schema_id',
+                FormSchema::query()
+                    ->withoutGlobalScope(OrganisationScope::class)
+                    ->where('organisation_id', $orgId)
+                    ->select('id'),
+            )
+            ->select('id');
+
+        $libraryIds = FormFieldLibrary::query()
+            ->withoutGlobalScope(OrganisationScope::class)
+            ->where('organisation_id', $orgId)
+            ->select('id');
+
+        $table = $model->getTable();
+
+        $builder->where(function (Builder $outer) use ($table, $fieldIds, $libraryIds): void {
+            $outer->where(function (Builder $q) use ($table, $fieldIds): void {
+                $q->where("$table.owner_type", 'form_field')
+                    ->whereIn("$table.owner_id", $fieldIds);
+            })->orWhere(function (Builder $q) use ($table, $libraryIds): void {
+                $q->where("$table.owner_type", 'form_field_library')
+                    ->whereIn("$table.owner_id", $libraryIds);
+            });
+        });
+    }
+
+    private function resolveOrganisationId(): ?string
+    {
+        if ($this->organisationId !== null) {
+            return $this->organisationId;
+        }
+
+        $route = request()->route();
+        if ($route === null) {
+            return null;
+        }
+
+        $org = $route->parameter('organisation');
+
+        if ($org instanceof \App\Models\Organisation) {
+            return $org->id;
+        }
+
+        if (is_string($org) && $org !== '') {
+            return $org;
+        }
+
+        $event = $route->parameter('event');
+        if ($event instanceof \App\Models\Event) {
+            return $event->organisation_id;
+        }
+
+        return null;
+    }
+}