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.).

nix/modules/default.nix

@@ -0,0 +1,8 @@
+{
+  gebos-ingester      = import ./gebos-ingester.nix;
+  gebos-supabase      = import ./gebos-supabase.nix;
+  gebos-caddy         = import ./gebos-caddy.nix;
+  gebos-postgres      = import ./gebos-postgres.nix;
+  gebos-hivemq        = import ./gebos-hivemq.nix;
+  gebos-frontend      = import ./gebos-frontend.nix;
+}