new: [piwigo] added ``backup`` relation.

Signed-off-by: Valentin Lab <valentin.lab@kalysto.org>

piwigo/metadata.yml

@@ -25,3 +25,8 @@ uses:
       location: !var-expand "$DATASTORE/$BASE_SERVICE_NAME/opt/apps/piwigo"
       # data-dirs: ## write permission for web-app
       #   - .
+  backup:
+    constraint: optional
+    auto: pair
+    solves:
+      backup: "Automatic regular backup of photos"