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/Enums/FormBuilder/FormFieldBindingMergeStrategy.php

@@ -0,0 +1,13 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Enums\FormBuilder;
+
+enum FormFieldBindingMergeStrategy: string
+{
+    case Overwrite = 'overwrite';
+    case Append = 'append';
+    case Replace = 'replace';
+    case FirstWriteWins = 'first_write_wins';
+}

api/app/Enums/FormBuilder/FormFieldBindingMode.php

@@ -0,0 +1,11 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Enums\FormBuilder;
+
+enum FormFieldBindingMode: string
+{
+    case EntityOwned = 'entity_owned';
+    case Mirrored = 'mirrored';
+}