feat(01-purchase-data-layer): add post-payment purchase data layer

Phase 1 complete:

- move purchase event to order confirmation after successful payment

- add backend purchase payload builder for transaction and ticket lines

- remove premature purchase push from przelewy24 redirect view

.paul/docs/API.md

@@ -0,0 +1,4 @@
+# API
+
+> Endpointy, kontrakty request/response, autentykacja.
+> Uzupelnij gdy projekt bedzie udostepnial API.