chore(infra): wire MLflow/Airflow env vars, fix healthcheck, add .dockerignore

- docker-compose: pass ML_SERVING_URL, MLFLOW_URL, AIRFLOW_URL + creds to api service
- docker-compose: pass NEXT_PUBLIC_MLFLOW_URL/AIRFLOW_URL to admin service
- docker-compose: replace wget healthcheck with node fetch (wget not in node image)
- docker-compose: enable Airflow basic_auth API backend; add MLflow pip dep for DAGs
- Dockerfiles: tighten layer caching, add .dockerignore

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,19 @@
+**/node_modules
+**/.next
+**/dist
+**/coverage
+**/.vitest-cache
+**/.turbo
+.git
+.gitea
+.github
+.vscode
+.idea
+**/.env
+**/.env.local
+**/*.log
+docs
+infra/docker/data
+**/__tests__
+**/*.test.ts
+**/*.test.tsx