The heredoc-based script writer in web.nix was indenting content inside
a nix '' string, which caused bash to prepend whitespace before the
#!/usr/bin/env bash shebang. The kernel rejected this as 'Exec format
error' (203/EXEC), preventing systemd from starting mcpedia-web.service.
Fix: use writeShellScriptBin instead of a raw heredoc. This ensures the
shebang starts at column 0 and uses the nix-resolved bash path.