infra: rongorongo state-sync backend + Traefik basicAuth

Add a Flask backend service for cross-device state persistence, storing
JSON state on an NFS volume at /mnt/docker-data/configs/rongorongo/data.
Frontend Dockerfile/nginx.conf reorganised into frontend/ subdirectory;
frontend API calls now route to /api/* handled by the backend container
over a dedicated bridge network. Traefik basicAuth middleware added to
the public URL with the bcrypt hash deliberately kept out of git
(REPLACE_AT_DEPLOY placeholder in rongorongo.yml). Prometheus probe moved
from https-external to internal http://192.168.86.192:8181/health to
bypass the new auth wall.

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

compose/hf-pdocker-01/rongorongo/frontend/Dockerfile

@@ -0,0 +1,14 @@
+FROM nginx:1.25-alpine
+
+RUN rm -rf /usr/share/nginx/html/*
+
+COPY index.html /usr/share/nginx/html/index.html
+COPY nginx.conf /etc/nginx/conf.d/default.conf
+RUN chmod 644 /usr/share/nginx/html/index.html
+
+HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
+  CMD wget -qO- http://127.0.0.1/health || exit 1
+
+EXPOSE 80
+
+CMD ["nginx", "-g", "daemon off;"]