feat: make boarding point price levelling switchable

docs/buspro-database-reference.md

@@ -225,6 +225,17 @@ JOIN dbo.ZustiegPreis zp ON zp.IDArtZustiegPreis_FS=lb.IDArtZustiegPreis_FS
 WHERE l.IDLeistung IN (@OutboundServiceId,@ReturnServiceId);
 ```
 
+**V 2026-09-28 — the XML export can omit the return-leg price.** Since one price row is
+surfaced twice, an asymmetric pair is an export defect: on products 701 and 2435 a
+`<zustieg_rueck>` carries no `preis` although the engine still bills the return
+`Abfahrtsortzuschlag`, so reading the export verbatim undercharges by one leg. Measured over
+all exports, a stated return price never differs from its outbound counterpart (510 pairs,
+zero counter-examples). `TravelParser` therefore levels a boarding point's price across both
+directions, switchable through `APP_BPN_LEVEL_BOARDING_POINT_PRICES` while the defect is open
+with the vendor. Flipping the flag needs `app:bpn:xml-cache-invalidate` and
+`app:bpn:refresh-travel-snapshot`. A point appearing **only** in `zustiege_rueck` without a
+price cannot be priced from our side — that is a catalog job.
+
 **N — the departure surcharge is computed.** Booking responses carry a position
 `art="SON" unterart="TRA"` (*Abfahrtsortzuschlag*) with no matching row type: `ArtZuschlag`
 — the lookup behind `BuchungTNZuschlag` — holds only discounts and fees