feat(billing): free is a subscription too, and the product says which plan

Every team now exists with the payment provider, free ones included. An upgrade
then changes a subscription rather than inventing a customer, and there is one
question to ask about anybody instead of two.

A subscription at nothing a month needs no payment, so it is created outright
rather than by sending somebody through a checkout to pay zero.

It runs after the team rows are committed and cannot affect them. Signing up is
not allowed to depend on a third party being reachable, so this cannot fail the
call and does not retry — a team that misses it is free, which is what it would
have been anyway, and the next call puts it right because the operation is
idempotent.

This broke the webhook mapping, which read the plan off the event type. A free
subscription announces itself with the same `subscription.created` a paid one
does, so every new signup would have landed on the paid allowance. The plan now
comes from the product, and a product we do not sell is left alone rather than
guessed at — somebody selling something else through the same account must not
be able to change what a team may run by doing so.

The external id stays the team. It is the billing subject, and keying on the
user would collapse somebody with two teams into one customer with no way to
say which subscription belonged to which.

.env.example

@@ -42,6 +42,9 @@ EMAIL_DEV_LOG=true
 POLAR_SERVER=sandbox
 POLAR_ACCESS_TOKEN=
 POLAR_PRODUCT_ID=
+# The product every team is put on at signup, priced at nothing. A free
+# subscription needs no checkout, so it is created outright.
+POLAR_FREE_PRODUCT_ID=
 # Signs every webhook delivery. Without it the webhook route refuses everything,
 # on purpose: nothing else stands in front of it.
 POLAR_WEBHOOK_SECRET=