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.).

.gitea/workflows/check.yml

@@ -0,0 +1,13 @@
+name: check
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+jobs:
+  flake-check:
+    runs-on: nix
+    steps:
+      - uses: actions/checkout@v4
+      - run: nix flake check
+      - run: nix build .#ingester .#frontend