feat: make email optional for under-16 participants except applicant

docs/bpn-xml-api-reference.md

@@ -167,11 +167,19 @@ fails on `Telefon-Mobil` and `Nationalität` because its `Vorgang_Details` recor
 `telefonprivat` and no `nationalitaet`.
 
 **Participants are held to less than the Anmelder** (measured 2026-09-16 on 2036, travel 12500, from a
-real step-3 `Anfrage`). Once a room is present, every `teilnehmer` needs a `Mail-Adresse`, children
-included: without `kommunikation` the answer is `650 … 1.Teilnehmer-Mail-Adresse, 2.Teilnehmer-Mail-Adresse`.
-`telefonmobil` and an empty `anschrift` pass for participants. An email identical to the Anmelder's is
-accepted. The field list above (Strasse, Plz, Ort, Telefon-Mobil for every participant) was measured on
-the placeholder records and is stricter than what BusPro applies to ordinary participants.
+real step-3 `Anfrage`). `telefonmobil` and an empty `anschrift` pass for participants. The field list
+above (Strasse, Plz, Ort, Telefon-Mobil for every participant) was measured on the placeholder records and
+is stricter than what BusPro applies to ordinary participants.
+
+**The participant `Mail-Adresse` is BusPro configuration, not a fixed rule.** Until 2026-10-13 every
+`teilnehmer` needed one once a room was present, children included: without it the answer was
+`650 … 1.Teilnehmer-Mail-Adresse, 2.Teilnehmer-Mail-Adresse`, while an email identical to the Anmelder's
+was accepted. On 2026-10-15 the requirement was switched off on the test system, and the same requests
+(no `email`, no `kommunikation`, an empty `<email/>`, adults and children, with and without a travelling
+Anmelder) all pass with an unchanged price. BusPro now accepts a participant without an email at any age,
+so the rule MyE&P applies (required from 16, and for the applicant at any age) is enforced only by
+`ParticipantDto` and `BookingDto` validation. Production needs the same configuration before under-16s
+without an email can be booked there.
 
 **The Anmelder does not have to travel.** An `<anmelder>` that matches no `teilnehmer` (a parent booking
 for their children) returns `möglich` with the same price as the control where the Anmelder is