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/gebos-frontend.nix

@@ -0,0 +1,19 @@
+{ config, lib, pkgs, ... }:
+
+let
+  cfg = config.services.gebos.frontend;
+in
+{
+  options.services.gebos.frontend = {
+    enable = lib.mkEnableOption "Gebos static SPA";
+    package = lib.mkOption {
+      type = lib.types.package;
+      default = pkgs.callPackage ../../frontend { };
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    # Nothing to install at the system level — Caddy serves the package's
+    # `share/frontend` directory directly. See nix/hosts/app-host.nix.
+  };
+}