refactor(#2862): swarm- prefix for the snapshot store

mara, in preparation for the swarm tier: the store is a swarm-level
role, not a hive one, so hive- was misleading about which tier it
belongs to. Module, units, syslog identifier, log lines and docs all
move to swarm-snapshot-store.

Also moved the option under services.hyperhive.swarm.snapshotStore, to
sit with swarm.peers and swarm.wireguard rather than dangling off the
top level. That is a judgement call beyond the literal rename — flagged
on the PR, and cheap precisely now: the option has never shipped, so
there is no deployment to migrate, whereas doing it after a release
would be a breaking change for no new benefit.

nix/host-modules/default.nix

@@ -19,9 +19,9 @@
     ./hive-matrix.nix
     ./hive-network.nix
     ./hive-priv.nix
-    ./hive-snapshot-store.nix
     ./hive-tls.nix
     ./otel.nix
+    ./swarm-snapshot-store.nix
     ./swarm-wireguard.nix
     ./swarm.nix
   ];