- Auto-detect H.264 encoder on startup: NVENC (NVIDIA) → QSV (Intel) →
  AMF (AMD) → VideoToolbox (macOS) → libx264 software fallback. A short
  test encode against each probes actual usability (not just `ffmpeg
  -encoders` advertisement).
- Transcoding now uses the best available encoder automatically — no
  configuration needed. Typical speedup: 5-20x over software.
- Startup log line shows which encoder is active.
- Desktop status bar shows an ⚡ / 🐢 icon with a tooltip that tells
  the user exactly what to install if no HW encoder is detected.
- GET /api/hwenc exposes the detected encoder.

Also: thumbnail requests are now deduplicated (same-episode concurrent
requests wait for the first one) and globally capped at 2 concurrent
ffmpegs, so opening a series page doesn't spawn 20 ffmpegs.