fix(nixos): remove hardcoded ACTIVITY_DB_PATH from digest timer

The CLI uses a relative default (data/activity.sqlite) which resolves
correctly from the WorkingDirectory (/srv/forms-lab/main). The hardcoded
path pointed to a non-existent location.

Author: danielnaab

infrastructure/nixos/modules/activity-digest.nix

@@ -16,7 +16,6 @@
     };
 
     script = ''
-      export ACTIVITY_DB_PATH="/srv/forms-lab/data/activity.sqlite"
       exec ${pkgs.bun}/bin/bun run src/entrypoints/cli/main.ts activity digest
     '';
   };