Mongo is a one-member replica set

Owner decision 2026-10-07: production's mongod becomes a one-member replica set (rs0), so that a backup can read every
collection at one instant (a snapshot read session), which a standalone mongod can't. The unit runs mongod with
--replSet rs0 and a 990 MB oplog; setup.sh converts it once, idempotently (PrivaPub stopped, mongod restarted,
rs.initiate, the primary awaited, PrivaPub started); every connection string says directConnection=true, which works
against the standalone too, so this code can deploy before the conversion. The CI's throwaway mongod and the pasture's
are replica sets as well, so the snapshot path is what the tests exercise; MongoTopology tells which a mongod is, and
TopologyTests holds the test mongod to it. The suite passes on it (906).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

tools/pasture/lib/pasture.sh

@@ -21,7 +21,11 @@ pasture_base_up() {
 	podman volume exists pasture-caddy-data || podman volume create pasture-caddy-data >/dev/null
 	# (PrivaPub's uploads live in a volume: a container made again must not lose them)
 	podman volume exists pasture-privapub-media || podman volume create --label pasture=1 pasture-privapub-media >/dev/null
-	podman run -d --replace --name pasture-mongo --network $net --network-alias mongo docker.io/library/mongo:8 --quiet >/dev/null
+	# a one-member replica set, as production's, so backups read at one instant
+	podman run -d --replace --name pasture-mongo --network $net --network-alias mongo docker.io/library/mongo:8 --quiet --replSet rs0 >/dev/null
+	for _ in $(seq 1 60); do podman exec pasture-mongo mongosh --quiet --eval 'db.hello()' >/dev/null 2>&1 && break; sleep 1; done
+	podman exec pasture-mongo mongosh --quiet --eval "rs.initiate({_id: 'rs0', members: [{_id: 0, host: 'mongo:27017'}]})" >/dev/null
+	for _ in $(seq 1 60); do [ "$(podman exec pasture-mongo mongosh --quiet --eval 'db.hello().isWritablePrimary')" = "true" ] && break; sleep 1; done
 	caddy_up
 	local extra=()
 	for setting in ${PRIVAPUB_ENV:-}; do extra+=(-e "$setting"); done