Add infrastructure skeleton for review (refs #21)

Lays out the agreed shape from the design discussion: one flake at the
root with three nixosConfigurations (mqtt-ingest, db-host, app-host),
vendored Supabase docker-compose pointed at the external db-host,
Caddy → Kong → PostgREST/GoTrue, a Go MQTT→Postgres ingester stub,
deploy-rs node map, and a Gitea Actions workflow that deploys in
db → app → ingest order on push to main.

No real implementation yet — every host module has TODOs marking the
gaps (real hardware config, actual hostnames, HiveMQ packaging,
vendor the full upstream Supabase compose, etc.).

frontend/default.nix

@@ -0,0 +1,21 @@
+{ buildNpmPackage, lib }:
+
+buildNpmPackage {
+  pname = "gebos-frontend";
+  version = "0.0.0";
+  src = ./.;
+
+  # Filled in once package-lock.json exists.
+  npmDepsHash = lib.fakeHash;
+
+  installPhase = ''
+    runHook preInstall
+    mkdir -p $out/share/frontend
+    cp -r dist/* $out/share/frontend/
+    runHook postInstall
+  '';
+
+  meta = {
+    description = "Gebos static SPA";
+  };
+}