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

compose.yml

@@ -120,7 +120,7 @@ x-minio-env: &minio-env
 
 services:
   app:
-    image: lasuite/impress-frontend:v6.0.3
+    image: lasuite/impress-frontend:v6.0.1
     networks:
       - backend
     deploy:
@@ -137,7 +137,7 @@ services:
       start_period: 10s
 
   backend:
-    image: lasuite/impress-backend:v6.0.3
+    image: lasuite/impress-backend:v6.0.1
     networks:
       - backend
     environment:
@@ -172,7 +172,7 @@ services:
       - jwt_pk
 
   celery:
-    image: lasuite/impress-backend:v6.0.3
+    image: lasuite/impress-backend:v6.0.1
     networks:
       - backend
     healthcheck:
@@ -203,7 +203,7 @@ services:
 
   # Collaboration server: serves the websocket and holds the document content
   yhub:
-    image: lasuite/impress-yhub:v6.0.3
+    image: lasuite/impress-yhub:v6.0.1
     networks:
       - backend
     environment: *yhub-env
@@ -244,7 +244,7 @@ services:
 
   # Conversion service only since v6 (no longer serves the collaboration)
   y-provider:
-    image: lasuite/impress-y-provider:v6.0.3
+    image: lasuite/impress-y-provider:v6.0.1
     networks:
       - backend
     healthcheck:
@@ -343,7 +343,7 @@ services:
       - minio_ru
 
   web:
-    image: nginx:1.31.8
+    image: nginx:1.31.7
     healthcheck:
       test: ["CMD", "curl", "-f", "http://localhost:8083"]
       interval: 15s