Create a Stripe Checkout link for enterprise/custom billing.

This generates a shareable checkout URL where:

• The customer email is pre-filled and locked
• X days of free trial are shown
• $Y/month is shown as the recurring price after trial
• The customer enters their card and subscribes

When the customer completes checkout:

• The existing handle_checkout_completed webhook fires
• The plan is activated automatically in the DB
• If a Stripe trial was set, they won't be charged until trial ends

Optionally, grant_trial_access=true (default) also gives DB-level plan access immediately so the user can start using the service before completing payment.

Flow for the admin:

1. Create checkout link via this endpoint
2. Send the URL to the customer (email, Slack, etc.)
3. Customer clicks link, sees trial + price, enters card
4. Plan activates automatically