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/app/Models/Company.php

@@ -5,10 +5,10 @@ declare(strict_types=1);
 namespace App\Models;
 
 use App\Models\Scopes\OrganisationScope;
+use Illuminate\Database\Eloquent\Builder;
 use Illuminate\Database\Eloquent\Concerns\HasUlids;
 use Illuminate\Database\Eloquent\Factories\HasFactory;
 use Illuminate\Database\Eloquent\Model;
-use Illuminate\Database\Eloquent\Builder;
 use Illuminate\Database\Eloquent\Relations\BelongsTo;
 use Illuminate\Database\Eloquent\Relations\HasMany;
 use Illuminate\Database\Eloquent\SoftDeletes;
@@ -21,13 +21,14 @@ final class Company extends Model
 
     protected static function booted(): void
     {
-        static::addGlobalScope(new OrganisationScope());
+        self::addGlobalScope(new OrganisationScope);
     }
 
     protected $fillable = [
         'organisation_id',
         'name',
         'type',
+        'kvk_number',
         'contact_first_name',
         'contact_last_name',
         'contact_email',