Set explicit Kopia UI credentials for web login.

Use HTTP basic auth username kopia with KOPIA_REPOSITORY_PASSWORD; document in README.

stacks/backup/init-kopia.sh

@@ -44,4 +44,7 @@ else
   echo "init-kopia: B2 credentials empty — start UI only; configure repo manually (see README)"
 fi
 
-exec kopia server start --ui --insecure --address=0.0.0.0:51515
+# Web UI uses HTTP basic auth (distinct from repository encryption password).
+exec kopia server start --ui --insecure --address=0.0.0.0:51515 \
+  --server-username=kopia \
+  --server-password="${KOPIA_PASSWORD}"