Completes residency end-to-end: a 'Gehört zum eigenen Bestand' checkbox on the
Rennmaus-Formular (default on for new animals) lets the user mark a tier as an
external ancestor by hand — previously isResident was only set at import. Wired
through FormState + formFromGerbil (defaults true) + the CreateGerbil payload.
German via de.ts (LF). e2e: edit Krümel, untick Bestand, save -> detail shows
the 'Extern' badge (proves form field -> payload -> persistence). Gate green:
build, eslint, vitest 78, e2e 86.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
