Fix storefront customer signup creation

[roncodes]

Summary

• ensure Storefront signup persists customer user type outside guarded mass assignment
• reuse existing customer contacts by company/user/type before creating a new contact
• return the identity conflict error instead of dereferencing a null customer after ContactObserver rejects creation

Root cause

Storefront attempted to mass-assign users.type = customer, but type is guarded on the core User model. Contact creation then triggered FleetOps customer identity validation and could throw UserAlreadyExistsException; Storefront recovered by looking up a contact by phone only, which could return null and crash at $customer->uuid.

Validation

• php -l server/src/Http/Controllers/v1/CustomerController.php
• git diff --check

Notes

• Package Composer scripts were not run because this checkout does not have package-local server_vendor/bin or monorepo vendor/bin installed.
• No documentation or API specification changes are needed; the request/response contract is unchanged.