One image for the whole app: engine, API and dashboard on one port

The backend image now builds the SPA and serves it on the API's own origin
(fluksio/spa.py), so an instance is a single container with no proxy, DNS,
port 80 or .env: `docker compose up -d` with the new compose.yml runs it on
localhost:8080. It starts on fluksio serve (persisted secret key, first
admin printed once) and brings its own Redis when REDIS_HOST is unset.

Slimmer: python:3.13-slim, no dev group, uv once, fastapi without the cloud
CLI. Optional features are per-feature extras (mqtt, influx, email, sentry,
ml, server for all) picked with the EXTRAS build argument; numpy moved to
`ml`. ffmpeg left the image for imageio-ffmpeg via the Modules screen.

The operator stack runs the same image with app. and api. routed to it; the
nginx frontend service, the LAN nginx proxy and DOMAIN in the backend are
gone, and the MCP issuer is FRONTEND_HOST.

docker/compose.local.yml

@@ -33,13 +33,6 @@ services:
       # talks to this backend rather than running one of its own.
       - BACKEND_CORS_ORIGINS=http://${DOMAIN:-localhost},http://app.${DOMAIN:-localhost},http://localhost:5173
 
-  frontend:
-    ports: !reset []
-    build:
-      args:
-        - VITE_API_URL=http://api.${DOMAIN:-localhost}
-        - NODE_ENV=development
-
 networks:
   # Shared with the website stack; created by the root scripts/setup.sh.
   proxy: