Add persistent volume for ~/Arbeitsblaetter (learning units, QA, audio cache)

lehrer_arbeitsblaetter volume mounted at /root/Arbeitsblaetter.
Survives container restarts — learning units, QA items, translations,
and audio cache are no longer lost on rebuild.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -20,6 +20,7 @@ volumes:
   transcription_models:
   transcription_temp:
   lehrer_backend_data:
+  lehrer_arbeitsblaetter:
   opensearch_data:
   # Communication (Jitsi + Matrix)
   synapse_data:
@@ -159,6 +160,7 @@ services:
     volumes:
       - /var/run/docker.sock:/var/run/docker.sock:ro
       - lehrer_backend_data:/app/data
+      - lehrer_arbeitsblaetter:/root/Arbeitsblaetter
     environment:
       PORT: 8001
       DATABASE_URL: postgresql+asyncpg://${POSTGRES_USER:-breakpilot}:${POSTGRES_PASSWORD:-breakpilot123}@bp-core-postgres:5432/${POSTGRES_DB:-breakpilot_db}?options=-csearch_path%3Dlehrer,core,public