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

abra.sh

@@ -1,8 +1,8 @@
 # Set any config versions here
 # Docs: https://docs.coopcloud.tech/maintainers/handbook/#manage-configs
-export ABRA_ENTRYPOINT_VERSION=v5
-export NGINX_CONF_VERSION=v3
-export PG_BACKUP_VERSION=v4
+export ABRA_ENTRYPOINT_VERSION=v6
+export NGINX_CONF_VERSION=v4
+export PG_BACKUP_VERSION=v5
 export MINIO_INITIALIZE_VERSION=v2
 export MIGRATE_VERSION=v1
 
@@ -18,3 +18,11 @@ migrate() {
 minio_initialize() {
   /minio-initialize.sh
 }
+
+# Hand all pre-v6 documents from the media bucket over to the collaboration
+# server (yhub). Resumable and safe to re-run; extra args are passed through,
+# e.g. --dry-run, --retry-failed, --concurrency N
+migrate_documents() {
+  source /abra-entrypoint.sh -e
+  python manage.py migrate_documents "$@"
+}