tests: exercise both nginx and caddy proxy backends

Parameterizes tests/common.nix and tests/default.nix on proxyServer
(default "nginx"), giving flake.nix two nixosConfigurations
(test-nginx/test-caddy) and `just build-vm [proxy]` a way to pick
between them for interactive testing.

The automated nixosTest now boots one node per proxy backend and runs
the full assertion suite (factored into check_invoiceninja()) against
each, so a regression in either nginx or caddy's config gets caught.

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

justfile

@@ -18,10 +18,10 @@ clean:
     pkill qemu
     exit 0
 
-[doc('Build virtual machine for testing')]
+[doc('Build virtual machine for testing (proxy: nginx or caddy)')]
 [group('main')]
-build-vm: clean
-    nom build ".#nixosConfigurations.test.config.system.build.vm"
+build-vm proxy="nginx": clean
+    nom build ".#nixosConfigurations.test-{{proxy}}.config.system.build.vm"
 
 [doc('Run virtual machine in current terminal')]
 [group('main')]