feat: add whisper.cpp ROCm backend support for AMD GPU acceleration

- transcription.py: new _transcribe_remote_whispercpp() using /inference endpoint
- transcription.py: backend param routes to openai or whispercpp remote path
- config.py: whisper.backend default 'openai', alt 'whispercpp'
- pipeline.py: passes backend from config to transcribe_file
- settings: backend dropdown (OpenAI-compat / whisper.cpp)
- SETUP.md: whisper.cpp ROCm build and systemd setup instructions

whisper-cpp-server running on beastix :8080 (ROCm0, gfx1030, RX 6800 XT)

config.py

@@ -13,6 +13,7 @@ DEFAULTS = {
         "language": "de",
         "device": "auto",  # "auto" = use GPU if ROCm available, else CPU
         "base_url": "",
+        "backend": "openai",  # "openai" = OpenAI-compatible API, "whispercpp" = whisper.cpp /inference
     },
     "audio": {
         "device": "",