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]>

frontend/src/viewer/components/VideoPlayer.tsx

@@ -529,14 +529,6 @@ const VideoPlayer = forwardRef<VideoPlayerHandle, VideoPlayerProps>(function Vid
 
       {/* Persistent live badge — the stream state should be readable without
           hovering, since "is this live?" is the first question a viewer asks. */}
-      {live && (
-        <div className="pointer-events-none absolute left-3 top-3 flex items-center gap-2">
-          <span className="flex items-center gap-1.5 rounded-full bg-black/60 px-2.5 py-1 text-xs font-semibold text-white backdrop-blur">
-            <span className="h-1.5 w-1.5 rounded-full bg-red-500" />
-            LIVE
-          </span>
-        </div>
-      )}
 
       {loading && (
         <div className="pointer-events-none absolute inset-0 flex items-center justify-center">