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 Delete the two superseded Briefwechsel design specs.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>
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