fix(checkout): prevent epoch membership expiration

[superdav42]

Summary

• use the calculated next charge date when recurring billing starts immediately
• preserve null expiration dates for lifetime memberships
• cover persisted, email-placeholder, and customer-facing expiration output

Root cause

Recurring carts without trials report a billing start timestamp of 0. Formatting that value directly persisted a January 1970 membership expiration, which then propagated to email event payloads and customer views.

Verification

• vendor/bin/phpunit --no-coverage --filter 'WP_Ultimo\\Checkout\\Checkout_Test' — 204 tests, 349 assertions, 1 skip
• vendor/bin/phpcs inc/checkout/class-checkout.php tests/WP_Ultimo/Checkout/Checkout_Test.php
• vendor/bin/phpstan analyse inc/checkout/class-checkout.php tests/WP_Ultimo/Checkout/Checkout_Test.php --no-progress
• git diff --check

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