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.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/scripts/capture-screenshots.mjs

@@ -124,27 +124,19 @@ async function captureRuns(page, dir) {
   await page.waitForTimeout(1500)
   await page.screenshot({ path: `${dir}/app-runs-compare.png`, fullPage: true })
 
-  // The API is its own host here, unless the stack serves it under the SPA's
-  // own origin (a LAN stack), whose CSP then refuses the other host.
-  const apiUrl = APP_URL.replace("//app.", "//api.")
-  const results = await page.evaluate(
-    async (bases) => {
-      for (const base of bases) {
-        const answer = await fetch(`${base}/api/v1/dashboards/`, {
-          headers: {
-            Authorization: `Bearer ${localStorage.getItem("access_token")}`,
-          },
-        }).catch(() => null)
-        if (!answer?.ok) continue
-        const body = await answer.json()
-        return (body.data || [])
-          .map((one) => one.name)
-          .find((n) => n.endsWith("_results"))
-      }
-      return null
-    },
-    [apiUrl, ""],
-  )
+  // The API is a path on the SPA's own origin.
+  const results = await page.evaluate(async () => {
+    const answer = await fetch("/api/v1/dashboards/", {
+      headers: {
+        Authorization: `Bearer ${localStorage.getItem("access_token")}`,
+      },
+    }).catch(() => null)
+    if (!answer?.ok) return null
+    const body = await answer.json()
+    return (body.data || [])
+      .map((one) => one.name)
+      .find((n) => n.endsWith("_results"))
+  })
 
   // A dashboard read against two runs: the same page a live run is watched
   // on, showing finished ones. This is the seam the feature exists for — and