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>

dev-docs/SCHEMA.md

@@ -1,10 +1,17 @@
 # Crewli — Core Database Schema
 
 > Source: Design Document v1.3 — Section 3.5  
-> **Version: 2.7** — Updated April 2026
+> **Version: 2.8** — Updated April 2026
 >
 > **Changelog:**
 >
+> - v2.8: WS-6 session 3a.5 — `companies.kvk_number` column added
+>   (nullable, indexed). Aligns with the binding-target registry's
+>   B2B identity-key candidate. Registry entries renamed/removed in
+>   the same session to match real model columns; consistency test
+>   extended with model-existence + column-existence assertions.
+>   RFC-WS-6.md v1.2 §3 Q9 addendum.
+>
 > - v2.7: WS-6 session 2.5 — `form_schemas.default_crowd_type_id` column
 >   added (nullable; required at publish time for event_registration via
 >   RequiresDefaultCrowdType guard). Replaces the silent `oldest()`
@@ -909,13 +916,14 @@ $effectiveDate       = $shift->end_date          ?? $shift->timeSlot->date;
 | `organisation_id` | ULID FK            | → organisations                           |
 | `name`            | string             |                                           |
 | `type`               | enum               | `supplier\|partner\|agency\|venue\|other` |
+| `kvk_number`         | string nullable    | KvK registration number (Dutch Chamber of Commerce). Indexed. **v2.8 — WS-6 sessie 3a.5** |
 | `contact_first_name` | string nullable    |                                           |
 | `contact_last_name`  | string nullable    |                                           |
 | `contact_email`      | string nullable    |                                           |
 | `contact_phone`   | string nullable    |                                           |
 | `deleted_at`      | timestamp nullable | Soft delete                               |
 
-**Indexes:** `(organisation_id)`  
+**Indexes:** `(organisation_id)`, `(kvk_number)`  
 **Soft delete:** yes
 
 ---