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-hivemq.nix

@@ -0,0 +1,33 @@
+{ config, lib, pkgs, ... }:
+
+let
+  cfg = config.services.gebos.hivemq;
+in
+{
+  options.services.gebos.hivemq = {
+    enable = lib.mkEnableOption "HiveMQ CE single-node broker";
+    persistentSessions = lib.mkOption {
+      type = lib.types.bool;
+      default = true;
+    };
+    tlsPort = lib.mkOption {
+      type = lib.types.port;
+      default = 8883;
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    # TODO: package HiveMQ CE (download tarball + JRE wrapper) and wire as a
+    # systemd unit. Until then this module only declares options so dependent
+    # hosts can be evaluated.
+    systemd.services.gebos-hivemq = {
+      description = "HiveMQ CE";
+      wantedBy = [ "multi-user.target" ];
+      serviceConfig = {
+        Type = "simple";
+        ExecStart = "${pkgs.coreutils}/bin/true";  # TODO: real HiveMQ command
+        Restart = "on-failure";
+      };
+    };
+  };
+}