deploy: keep app data in /opt/bike_admin/data (ReadWritePaths)

Match the live volundr unit: drop StateDirectory=/var/lib/bike_admin in
favour of ReadWritePaths=/opt/bike_admin/data. systemd requires the
ReadWritePaths target to exist before the service starts, so the unit
documents the one-time install -d prerequisite.

deploy/bike_admin.service

@@ -21,6 +21,10 @@ NoNewPrivileges=true
 PrivateTmp=true
 ProtectSystem=full
 ProtectHome=true
+# App data lives next to the checkout at /opt/bike_admin/data (created by the
+# app on startup). systemd requires the ReadWritePaths target to exist before
+# it starts the service, so pre-create it with:
+#   sudo install -d -o bike -g bike /opt/bike_admin/data
 ReadWritePaths=/opt/bike_admin/data
 
 [Install]