Add an automated VM test against a real Invoice Ninja instance

Adds an invoiceninja-nixos flake input and two things built from it:

- nixosConfigurations.invoiceninja-test-nginx/-caddy: manual test
  VMs, re-exported as-is from that repo rather than redefined here —
  same principle as this crate being a client of the SDK, not a
  second implementation of it.
- checks.invoiceninja-cli: an automated nixosTest (tests/, adapted
  from invoiceninja-nixos's own test scaffolding) that boots a real
  Invoice Ninja instance and runs the actual invoiceninja-cli binary
  against it - create/list/get/update/invoice/payment/delete across
  clients, invoices, and payments - rather than raw curl.

tests/common.nix's module import had to change from a relative
../nixos-module/invoiceninja.nix (which doesn't exist here) to a
parameter threaded from the flake input's nixosModules.invoiceninja.

Building the VM pulls from the invoiceninja-nixos cachix cache via a
nixConfig block, mirroring that repo's own.

Also fixes a wrong assumption the first version of this test's final
assertion made, caught by actually running it against a real server:
`clients list` with no `--status` does not exclude soft-deleted
records - confirmed empirically (Invoice Ninja returns everything
regardless of is_deleted unless status=active is passed explicitly).
Corrected the test to check is_deleted via `clients get` instead, and
fixed the misleadingly-incomplete --status doc comment in cli.rs that
had the same wrong assumption baked in.

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

.gitignore

@@ -5,3 +5,6 @@
 /.cargo/config.toml
 /result
 /result-*
+# The test VM's disk image — created in the invocation directory by
+# run-nixos-vm (see CLAUDE.md's "Testing against a real server").
+/nixos.qcow2