Add Docker deployment (build- and stack-tested)

- backend/Dockerfile: gunicorn, not the dev server.
- frontend/Dockerfile: multi-stage, nginx serves the static build.
- nginx.conf proxies /api and /auth to the backend so both services
  share one origin - keeps the session cookie simple first-party,
  no SameSite=None/CORS complexity in production.
- docker-compose.yml: bind-mounts ./data/{instance,uploads} (not
  named volumes) so the sqlite db and uploaded photos are visible
  under the project dir; :Z flag for SELinux-enforcing hosts
  (Fedora/RHEL) - without it gunicorn fails with "unable to open
  database file".
- FRONTEND_PORT env var to pick the exposed port.
- .env.example: documents CORS_ORIGIN is moot in the compose setup
  (same-origin via nginx) and that SESSION_COOKIE_SECURE=true needs
  a TLS-terminating reverse proxy in front in real deployment.

Verified: both images build clean, full stack up via docker compose,
login flow starts for real against the actual NC instance through
the nginx proxy, sqlite db persists to the bind mount correctly.

docker-compose.yml

@@ -0,0 +1,23 @@
+services:
+  backend:
+    build: ./backend
+    env_file: ./backend/.env
+    volumes:
+      # Bind mounts (not named volumes) so the sqlite db and uploaded
+      # receipt photos live visibly under ./data, not hidden inside
+      # Docker's own volume storage. `:Z` relabels them for the container on
+      # SELinux-enforcing hosts (Fedora/RHEL) - without it the container
+      # gets "unable to open database file" since the host dir's default
+      # user_home_t context isn't accessible to it. Harmless no-op on
+      # non-SELinux hosts.
+      - ./data/instance:/app/instance:Z
+      - ./data/uploads:/app/uploads:Z
+    restart: unless-stopped
+
+  frontend:
+    build: ./frontend
+    ports:
+      - "${FRONTEND_PORT:-8080}:80"
+    depends_on:
+      - backend
+    restart: unless-stopped