Release v2.15.2

fix(cloud): save pickup prefix mode from Stammdaten

Summary

• Move Abholcode-Buchstabe von into event Stammdaten (Config) so mode persists with Speichern
• Remove the Stationen/Kassen autosave bridge that left Station falling back to Kasse after reload
• Archive OpenSpec change pickup-prefix-mode-stammdaten and sync main pickup-prefix-mode spec

Test plan

• Cloud frontend unit tests
• Backend test_events_crud pickup prefix mode round-trip + pickup prefix mode suite
• Lint + typecheck
• Manual: Stammdaten → Station → Speichern → reload → still Station; Stationen shows letters; Kassen hides register letters