Task 7: the readiness collector (App\Support\Readiness) and its first
group, billing (App\Support\Readiness\BillingChecks). Reports missing
prerequisites — the active mode's Stripe key, its webhook signing
secret, complete company details, an invoice series per document kind,
a fully Stripe-synced catalogue — without enforcing anything itself;
the real locks (IssueInvoice, the checkout's Stripe-key guard) stay
where they are.
The webhook check uses App\Support\StripeWebhookSecret::current()
rather than querying config() by mode a second time, and the company
check calls CompanyProfile::missingForInvoicing() rather than
duplicating its list — two sources for one question is how they drift
apart.
Added lang/de/readiness.php and lang/en/readiness.php with every
`readiness.*` key this group uses, each `_breaks` sentence naming the
actual downstream failure rather than repeating the field name.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>