nixos/paperless: move paperless-manage to proper systemPackage

[SuperSandro2000]

Closes #343961

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[erikarvstedt]

This doesn't work on systems that use doas instead of sudo.
Ways to solve this:

• Add a simple runtime warning in paperless-manage that asks users to manually run the command with doas.

• Enable sudo in the paperless module. Unsafe and intrusive, not viable.

• Add doas support. I've implemented this here, feel free to squash.
  Because doas doesn't support sudo option --preserve-env, we sudo/doas-exec the whole paperless-manage script to run the env setup after sudo/doas.

  Note: Module btrbk has a similar sudo/doas check. It probably doesn't make sense yet to extract this to a common lib function.