feat(infra): add production Dockerfile and docker-compose.prod.yml

Multi-stage alpine-based build, healthcheck on /api/health, /data volume.
Verified end-to-end: image builds, container starts, health returns 200.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,10 @@
+node_modules
+.svelte-kit
+build
+data
+.git
+docs
+tests
+*.log
+.env
+.env.local