feat: split booking between participants and applicant

addresses #869f2q1vu

docs/bpn-xml-api-reference.md

@@ -208,7 +208,7 @@ the same way. Ask `MOEGLICHEAENDERUNGEN` first (`<idreise>`), which answers:
 | `buchungsart` | yes | yes | Edit **hardcodes `Buchung`** (`BookingClient::updateBooking()`) |
 | `status` | yes | yes | Edit replays the value read back from BusPro — see the caveat below |
 | `idreise`, `idpartner`, `idagentur` | yes | yes | `idpartner` sent unconditionally, even with no room |
-| `anmelder` | yes | yes | Create builds it from participant[0]; edit uses `PersonalData::toPayload()`. BusPro accepts an Anmelder who is not a participant |
+| `anmelder` | yes | yes | Create builds it from participant[0], or from `BookingDto::$applicant` when the applicant does not travel; edit uses `PersonalData::toPayload()`. BusPro accepts an Anmelder who is not a participant |
 | `anmelder/bemerkung` | — | yes | Via `PersonalData::toPayload()` |
 | `teilnehmerliste` | yes | yes | |
 | `wünsche` (`unterbringungswunsch`, `beförderungswunsch`) | yes | yes | |