1.2.0

What's new

• A payment carries lines, not one product. An order bump — a checkbox at
  checkout adding a second item — is now one payment with two lines:
  start(['noten-paket', 'uebungsblaetter']), or with quantities
  start(['noten-paket' => 1, 'uebungsblaetter' => 3]). The total is their sum,
  each line keeps the name it was sold under, and a line is never a second
  payment.
  All or none: a handle that is not in the catalogue refuses the whole
  checkout instead of quietly dropping the line, and two currencies in one
  payment are refused for the same reason.
  Done now rather than later on purpose: the schema change costs nothing while
  the addon has no installs, and would be a migration on other people's servers
  afterwards.
• Follow-up offers, off by default. An offer shown after a payment, charged
  without asking for card details a second time. docs/follow-up-offers.md is
  the whole story, and most of it is not technical: in Germany a follow-up order
  still needs its own unambiguously labelled button with the essential details
  directly above it. What is saved is the card number, not the consent.
• The offer disappears once taken. A second click, a double submit, a reloaded
  confirmation — all of them would otherwise charge again for the same thing. A
  refused charge does not count as taken.
• A follow-up is never treated as paid on acceptance. A recurring charge is
  accepted now and settled later; only the webhook decides, exactly as at
  checkout.
• The entitlements bridge grants every paid line, not only the first. A bump
  the buyer ticked and paid for is as bought as the thing they came for.

What's fixed

• A payment's lines are deleted with it even where the database does not enforce
  the foreign key — which on SQLite it quietly does not. Orphaned lines would
  have counted towards every revenue report ever run.