mealie: add python3 to image for the borgmatic SQLite dump helper

The borgmatic k8s-sqlite-dump helper runs python3 (stdlib sqlite3 .backup)
inside the pod, but the minimal Nix mealie image had no python3 on PATH, so
the mealie SQLite backup produced a 0-byte file. Add pkgs.python3 (same
nixpkgs build mealie uses, so negligible closure growth), matching shower.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

containers/mealie/default.nix

@@ -48,6 +48,10 @@ pkgs.dockerTools.buildLayeredImage {
     pkgs.coreutils
     pkgs.cacert
     pkgs.tzdata
+    # python3 (stdlib sqlite3) for the borgmatic k8s-sqlite-dump helper,
+    # which runs `python3 -c "...sqlite3...backup..."` inside the pod.
+    # Same nixpkgs python mealie is built against, so ~no added closure.
+    pkgs.python3
   ];
 
   config = {