back off the matrix daemon's restarts so a homeserver outage can't flood the store with logins

nix/agent-modules/matrix.nix

@@ -396,6 +396,13 @@ in
         # non-zero exit, which `on-failure` already restarts.
         Restart = "on-failure";
         RestartSec = 5;
+        # Each start logs in to the store before its first homeserver call, so
+        # a down homeserver must not restart it every `RestartSec`. systemd
+        # grows the delay exponentially from `RestartSec` to `RestartMaxDelaySec`
+        # over `RestartSteps` restarts and resets it after a successful run
+        # (systemd.service(5)).
+        RestartSteps = 6;
+        RestartMaxDelaySec = 300;
         # The daemon exits 75 (`ACCOUNTS_CHANGED_EXIT` in
         # hive-matrix-mcp/src/main.rs) when the store's linked accounts change:
         # a clean exit that must still restart it onto the new set.