Add storage keepalive (upholds) across services, Nextcloud cron, disable Nextcloud PHP JIT

Requires= doesn't restart a service once its required mount comes back after
a USB disconnect/remount, so a dedicated storage-keepalive unit now Upholds=
every storage-dependent service to bring it back automatically. Also adds the
missing Nextcloud cron.php timer (background jobs were never triggered), and
disables opcache.jit in the Nextcloud container — PHP 8.4's JIT was
miscompiling OC\Config\UserConfig::getValueBool(), crashing every
session-bootstrapping request with a bogus strtolower() TypeError.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

modules/services/mlflow.nix

@@ -118,6 +118,8 @@ in
     # -------------------------------------------------------------------------
     # Storage directories on external HD
     # -------------------------------------------------------------------------
+    homey.storage.upholds = [ "podman-mlflow.service" ];
+
     homey.storage.extraDirs = [
       { path = "mlflow"; }
       { path = "mlflow/data";      mode = "0750"; }