move taskchampion persistence to its own module

hosts/genepi/persistence.nix

@@ -22,8 +22,6 @@
       "/etc/ssh/ssh_host_ed25519_key.pub"
       # tailscale
       "/var/lib/tailscale/tailscaled.state"
-      # taskchampion
-      "/var/lib/taskchampion-sync-server/taskchampion-sync-server.sqlite3"
     ];
     users.rpqt = {
       directories = [ ];

hosts/genepi/taskchampion.nix

@@ -1,11 +1,15 @@
-{ config, ... }:
+{ config, lib, ... }:
 let
   domain = "home.rpqt.fr";
   subdomain = "tw.${domain}";
+  hasImpermanence = config.environment.persistence."/persist".enable;
 in
 {
   services.taskchampion-sync-server.enable = true;
 
+  services.taskchampion-sync-server.dataDir =
+    (lib.optionalString hasImpermanence "/persist") + "/var/lib/taskchampion-sync-server";
+
   services.nginx.virtualHosts.${subdomain} = {
     forceSSL = true;
     useACMEHost = "${domain}";