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/Configs/FormFieldConfigBackfillAndDropTest.php

@@ -30,7 +30,7 @@ final class FormFieldConfigBackfillAndDropTest extends TestCase
         // Roll back 4 WS-5c migrations + 2 WS-6 migrations + 5 WS-5b
         // migrations = 11, to get the pre-WS-5b state where the JSON column
         // still exists on form_fields / form_field_library.
-        $this->artisan('migrate:rollback', ['--step' => 13])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 14])->assertSuccessful();
         $this->assertTrue(Schema::hasColumn('form_fields', 'validation_rules'));
 
         $fieldId = $this->seedField([