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/database/migrations/2026_04_28_140000_add_kvk_number_to_companies_table.php

@@ -0,0 +1,38 @@
+<?php
+
+declare(strict_types=1);
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+/**
+ * Adds a nullable kvk_number column to companies. Crewli's binding-target
+ * registry references company.kvk_number as a B2B identity-key candidate
+ * (per WS-6 RFC binding registry); the column needed to exist on the
+ * model first.
+ *
+ * Nullable: not every Company has a registered KvK (foreign companies,
+ * partners, agencies). Identity-key validation runs at publish time per
+ * RequiresIdentityKeyBinding guard, not at the schema level.
+ *
+ * No backfill — pre-launch the table is short and seed-driven.
+ */
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('companies', function (Blueprint $table): void {
+            $table->string('kvk_number')->nullable()->after('type');
+            $table->index('kvk_number');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('companies', function (Blueprint $table): void {
+            $table->dropIndex(['kvk_number']);
+            $table->dropColumn('kvk_number');
+        });
+    }
+};