Adapt recipe to Docs v6 (yhub collaboration server)

- the new yhub service needs new initialisation and RSA keys
- SOFT_MIGRATION on by default, add migrate_documents command
- back up the yhub database too
- document the v5 -> v6 upgrade in README and release/next

Following the [C4DT AI Policy](https://git.coopcloud.tech/c4dt/ai-policy/src/branch/main/AI-POLICY.md):

 #LLM_vibe
 #LLM_model Claude Opus 5.5

- [x] I take full responsibility for this code
- [x] I understand every line of this code
- [x] I tested this code thoroughly on my own computer / servers
- [x] None of the commit messages were created using LLMs
- [x] I will participate in any discussion on my own and will not use an LLM for the discussion

pg_backup.sh

@@ -3,10 +3,16 @@
 set -e
 
 BACKUP_FILE='/var/lib/postgresql/data/pgdata/backup.sql'
+# database of the collaboration server (yhub), since v6
+YHUB_DB='yhub'
+YHUB_BACKUP_FILE='/var/lib/postgresql/data/pgdata/backup-yhub.sql'
 
 function backup {
   export PGPASSWORD=$(cat $POSTGRES_PASSWORD_FILE)
   pg_dump -U ${POSTGRES_USER} ${POSTGRES_DB} > $BACKUP_FILE
+  if psql -U ${POSTGRES_USER} -d postgres -tAc "SELECT 1 FROM pg_database WHERE datname='${YHUB_DB}'" | grep -q 1; then
+    pg_dump -U ${POSTGRES_USER} ${YHUB_DB} > $YHUB_BACKUP_FILE
+  fi
 }
 
 function restore {
@@ -27,6 +33,12 @@ function restore {
     createdb -U ${POSTGRES_USER} ${POSTGRES_DB}
     psql -U ${POSTGRES_USER} -d ${POSTGRES_DB} -1 -f $BACKUP_FILE
 
+    if [ -f $YHUB_BACKUP_FILE ]; then
+        psql -U ${POSTGRES_USER} -d postgres -c "DROP DATABASE IF EXISTS ${YHUB_DB} WITH (FORCE);"
+        createdb -U ${POSTGRES_USER} ${YHUB_DB}
+        psql -U ${POSTGRES_USER} -d ${YHUB_DB} -1 -f $YHUB_BACKUP_FILE
+    fi
+
     trap - EXIT INT TERM
     restore_config
 }