feat: encode on the GPU and restart individual renditions

Hardware encoding
- Detect the best usable encoder at startup by encoding a test frame with each
  candidate in turn (nvenc, qsv, amf, then libx264). Being compiled into ffmpeg
  is not enough: h264_amf fails without amfrt64.dll and h264_qsv fails without an
  Intel iGPU, so the probe is the only reliable answer.
- Translate the quality target into the chosen encoder's own vocabulary. libx264's
  -crf and "ultrafast" mean nothing to h264_nvenc, which takes -cq and p1..p7, so
  passing the configured values straight through would have been silently wrong.
- On the reference GTX 1060 a 1080p->720p rendition costs ~62% of a core on NVENC
  versus ~109% on libx264 ultrafast.

Per-rendition supervision
- Track encoder processes per rendition, not per stream. is_running answered
  "any alive", so when the source ("原画") encoder died while 1080p and 720p kept
  running, the supervisor saw a healthy stream and never restarted it — the
  source URL then 404'd permanently. It now requires every rendition to be alive
  and reports which ones are down.
- Restart only the dead renditions, so a single failure no longer interrupts
  viewers on the renditions that were fine.

Production serving
- Serve the built frontend from FastAPI with SPA fallback, so production is one
  process and one port with no Node runtime. Node becomes a build-time dependency.
- start.ps1/start.sh build the frontend on first run instead of failing to a 503.

Cleanup
- Drop the duplicated top-left LIVE badge; the control strip already carries it.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

start.ps1

@@ -1,5 +1,6 @@
 param(
     [switch]$InstallDeps,
+    [switch]$BuildFrontend,
     [string]$HostAddress = "0.0.0.0",
     [int]$BackendPort = 8080
 )
@@ -41,11 +42,29 @@ if ($InstallDeps) {
     Pop-Location
 }
 
+# The frontend is built once and then served by the backend, so Node is only
+# needed here — production never runs npm. Done automatically on a first start so
+# a fresh checkout does not come up as a 503 with no obvious cause.
+$FrontendDir = Join-Path $Base "frontend"
+$DistDir = Join-Path $FrontendDir "dist"
+if ($BuildFrontend -or -not (Test-Path (Join-Path $DistDir "index.html"))) {
+    if (Get-Command "npm" -ErrorAction SilentlyContinue) {
+        Write-Host "[StreamCore] Building frontend..."
+        Push-Location $FrontendDir
+        if (-not (Test-Path "node_modules")) { npm install }
+        npm run build
+        Pop-Location
+    } else {
+        Write-Warning "npm not found and frontend\dist is missing; the site will return 503."
+        Write-Warning "Install Node.js and run: cd frontend; npm install; npm run build"
+    }
+}
+
 # MediaMTX, the transcoders and the HLS tree are managed by the backend itself:
 # it starts the media origin at boot, supervises ffmpeg, and restarts either if it
 # dies. This launcher only has to bring up the process that owns all of that.
 Write-Host "[StreamCore] Starting backend on ${HostAddress}:${BackendPort} ..."
-Write-Host "[StreamCore] MediaMTX, transcoders and HLS are supervised by the backend."
+Write-Host "[StreamCore] Frontend, MediaMTX, transcoders and HLS are served by the backend."
 
 $backendArgs = @("-m", "uvicorn", "main:app", "--host", $HostAddress, "--port", "$BackendPort")
 & $Python @backendArgs