nixos/castopod: fix startup, displaying images, uploads up to 500 MiB

- new maxUploadSize option
- new dataDir option (with ReadWritePaths systemd support)
- admin page reports correct free disk space (instead of /nix/store)
- fix example configuration in documentation
- now podcast creation and file upload are tested during NixOS test
- move castopod from audio to web-apps folder
- verbose logging from the browser test

nixos/modules/module-list.nix

@@ -342,7 +342,6 @@
   ./services/amqp/rabbitmq.nix
   ./services/audio/alsa.nix
   ./services/audio/botamusique.nix
-  ./services/audio/castopod.nix
   ./services/audio/gmediarender.nix
   ./services/audio/gonic.nix
   ./services/audio/goxlr-utility.nix
@@ -1302,6 +1301,7 @@
   ./services/web-apps/bookstack.nix
   ./services/web-apps/c2fmzq-server.nix
   ./services/web-apps/calibre-web.nix
+  ./services/web-apps/castopod.nix
   ./services/web-apps/coder.nix
   ./services/web-apps/changedetection-io.nix
   ./services/web-apps/chatgpt-retrieval-plugin.nix