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/Providers/AppServiceProvider.php

@@ -48,6 +48,7 @@ use App\Models\VolunteerAvailability;
 use App\Events\FormBuilder\FormSubmissionSubmitted;
 use App\Listeners\FormBuilder\SyncTagPickerSelectionsOnSubmit;
 use App\Listeners\FormBuilder\TriggerPersonIdentityMatchOnFormSubmit;
+use App\Observers\FormBuilder\FormFieldBindingsCascadeObserver;
 use App\Observers\FormBuilder\FormSubmissionObserver;
 use App\Observers\FormBuilder\FormValueObserver;
 use App\Observers\PersonObserver;
@@ -94,6 +95,11 @@ class AppServiceProvider extends ServiceProvider
         FormValue::observe(FormValueObserver::class);
         \App\Models\FormBuilder\FormSubmission::observe(FormSubmissionObserver::class);
 
+        // Cascade binding rows on owner delete (WS-5a). Bindings are physical
+        // state; deleted on soft-delete as well as hard-delete of the owner.
+        FormField::observe(FormFieldBindingsCascadeObserver::class);
+        FormFieldLibrary::observe(FormFieldBindingsCascadeObserver::class);
+
         // ARCH §31.10 — FORM-02 TAG_PICKER sync listener.
         \Illuminate\Support\Facades\Event::listen(
             FormSubmissionSubmitted::class,