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>

.gitignore

@@ -0,0 +1,3 @@
+target/
+.env
+kanidm-data/