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.

compose.yml

@@ -0,0 +1,35 @@
+# Fluksio in one container. From a clone of this repository:
+#
+#   docker compose up -d
+#   docker compose logs fluksio    # the admin password, printed once
+#
+# then open http://localhost:8080. Change the left side of `ports` to use
+# another host port. The operator stack under docker/ is a different file.
+name: fluksio
+
+services:
+  fluksio:
+    build:
+      context: .
+      dockerfile: backend/Dockerfile
+      args:
+        # Optional features, comma separated: mqtt, influx, email, sentry, ml,
+        # or server for all of them. Rebuild after a change: `docker compose up -d --build`.
+        EXTRAS: ""
+    image: fluksio:latest
+    restart: unless-stopped
+    ports:
+      - "8080:8000"
+    volumes:
+      - data:/data
+    environment:
+      # The address people and devices reach this instance at. Panel links and
+      # mails are built from it.
+      FRONTEND_HOST: http://localhost:8080
+      # Schedules run in this timezone, such as Europe/Berlin.
+      TZ: UTC
+    # Where frames a flow only shows live are held.
+    shm_size: 128mb
+
+volumes:
+  data: