feat(companies): add kvk_number column for B2B identity binding (WS-6)

WS-6 binding-target registry references company.kvk_number as a B2B
identity-key candidate. The column needed to exist on the model
before the registry could legitimately reference it. Nullable
because not every Company has a registered KvK (foreign companies,
partners, agencies); identity-key publish guards enforce presence
where required, not at schema level.

Changes:
- New migration `2026_04_28_140000_add_kvk_number_to_companies_table`
  adds nullable string column + index after `type`.
- Company::$fillable expanded.
- CompanyFactory generates an 8-digit KvK by default.
- CompanyKvkNumberTest covers attribute persistence, nullability,
  and information_schema-verified index existence.
- SCHEMA.md → v2.8 with the new column row + indexes line.
- Schema dump regenerated (CI fast-path).

Migration step counts in 5 backfill tests bumped +1 (the new
migration sits at the top of the migration stack):
  - FormFieldBindingMigrationTest:           18→19, 16→17
  - ConditionalLogicBackfillTest:             7→8
  - FormFieldConfigBackfillAndDropTest:      13→14
  - FormFieldOptionsBackfillTest:             3→4
  - FormFieldValidationRuleBackfillTest:     16→17

Refs: WS-6 sessie 3a binding-target drift audit

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

api/tests/Feature/FormBuilder/Bindings/FormFieldBindingMigrationTest.php

@@ -57,7 +57,7 @@ final class FormFieldBindingMigrationTest extends TestCase
         // validation-rules-backfill, create-validation-rules) +
         // 2 WS-6 migrations (action-failures, apply-status) +
         // 2 WS-5a migrations (drop-binding-cols, create-bindings) = 16.
-        $this->artisan('migrate:rollback', ['--step' => 18])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 19])->assertSuccessful();
         $this->assertFalse(Schema::hasTable('form_field_bindings'));
         $this->assertTrue(Schema::hasColumn('form_fields', 'binding'));
         $this->assertTrue(Schema::hasColumn('form_field_library', 'default_binding'));
@@ -119,7 +119,7 @@ final class FormFieldBindingMigrationTest extends TestCase
     public function test_rollback_reconstructs_json_and_drops_table(): void
     {
         // Walk back the full WS-5d + WS-5c + WS-6 + WS-5b + WS-5a stack (16 migrations).
-        $this->artisan('migrate:rollback', ['--step' => 18])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 19])->assertSuccessful();
         [$fieldAId] = $this->seedFieldsWithBindingJson();
         [$libAId] = $this->seedLibraryWithBindingJson();
 
@@ -134,7 +134,7 @@ final class FormFieldBindingMigrationTest extends TestCase
         // the pre-WS-5b state (conditional-logic, validation-rules, configs
         // and options tables gone, validation_rules + options JSON columns
         // reappear on source tables; binding contract intact).
-        $this->artisan('migrate:rollback', ['--step' => 16])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 17])->assertSuccessful();
         $this->assertFalse(Schema::hasTable('form_field_options'));
         $this->assertFalse(Schema::hasTable('form_field_conditional_logic_groups'));
         $this->assertFalse(Schema::hasTable('form_field_conditional_logic_conditions'));