cnats: NATS-native chat with Leptos SSR and Kanidm SSO

Initial import plus deployment packaging: multi-stage Dockerfile
(cargo-leptos build -> debian-slim runtime), .dockerignore, and a
dev-only docker-compose (app + local NATS). Production deployment
lives in the infrastructure repo.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docker-compose.yml

@@ -0,0 +1,21 @@
+# Local development stack. In production the app is deployed by the
+# infrastructure repo (terraform), which supplies NATS and Kanidm — this
+# compose file only exists for standalone hacking.
+
+services:
+  app:
+    build: .
+    env_file: .env
+    environment:
+      NATS_URL: nats://nats:4222
+    ports:
+      - "3000:3000"
+    depends_on:
+      - nats
+
+  nats:
+    image: nats:2.10-alpine
+    command: ["-js", "-m", "8222"]
+    ports:
+      - "4222:4222"   # client connections
+      - "8222:8222"   # monitoring UI (http://localhost:8222)