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.dev.yml

@@ -147,17 +147,6 @@ services:
       - "1080:1080"
       - "1025:1025"
 
-  frontend:
-    restart: "no"
-    ports:
-      - "5173:80"
-    build:
-      context: ..
-      dockerfile: frontend/Dockerfile
-      args:
-        - VITE_API_URL=http://localhost:8000
-        - NODE_ENV=development
-
   # Reachable at http://docs.${DOMAIN}, but a container rebuild per edit is a
   # poor authoring loop — use `make docs-serve` for that.
   docs: