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>

tests/common.nix

@@ -1,3 +1,4 @@
+{ proxyServer ? "nginx" }:
 { ... }:
 
 {
@@ -5,7 +6,7 @@
 
   services.invoiceninja = {
     enable = true;
-    proxy.server = "nginx";
+    proxy.server = proxyServer;
     adminAccount.passwordFile = ./invoice_ninja_test_password;
     secretFile = ./test.env;
     settings.APP_DEBUG = true;