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.).

ingester/go.mod

@@ -0,0 +1,7 @@
+module github.com/gebos/ingester
+
+go 1.22
+
+// TODO after first `go mod tidy`:
+//   - github.com/eclipse/paho.mqtt.golang
+//   - github.com/jackc/pgx/v5