docs: remove Briefwechsel from architecture, routes and glossary

Drop the Briefwechsel route and the conversation derived-domain /
conversation-thread prose from the route tables (CLAUDE.md,
frontend/CLAUDE.md), ARCHITECTURE.md, the C4 frontend/backend diagrams,
and GLOSSARY.md (term + derived-domain list). Delete the two superseded
Briefwechsel design specs. Historical ADRs and dated analyses are left
untouched as point-in-time context.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/CLAUDE.md

@@ -29,7 +29,6 @@ src/
 │   ├── +page.svelte     # Home / document search dashboard
 │   ├── documents/       # Document CRUD, detail, edit, upload
 │   ├── persons/         # Person directory (filtered, paginated), detail, edit, merge, review (triage)
-│   ├── briefwechsel/    # Bilateral conversation timeline
 │   ├── aktivitaeten/    # Unified activity feed (Chronik)
 │   ├── admin/           # User, group, tag, OCR, system management
 │   ├── api/             # Internal API proxies (server-side only)