Lifts the three-API-call seeding (create sender, create receiver,
create document) out of briefwechsel-a11y.spec.ts and into a
dedicated fixtures module. The spec now calls seedBilateralPair()
in beforeAll and cleanupBilateralPair() in afterAll so the test
DB doesn't accrue seeded rows across reruns.
Two caveats captured in the helper docstring: the backend has no
person-delete endpoint (only the document is purged), and the
timestamped last names make leftover persons collision-free.
Refs #305
Fixes @saraholt follow-up 1 + 2 from PR round-2 review
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
0e988a9d42