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)

frontend/settings.html

@@ -74,9 +74,16 @@
 
     <section>
       <h2>Verarbeitung</h2>
+      <div class="field">
+        <label>Whisper Backend</label>
+        <select id="whisper-backend">
+          <option value="openai">OpenAI-kompatibel (faster-whisper-server)</option>
+          <option value="whispercpp">whisper.cpp Server</option>
+        </select>
+      </div>
       <div class="field">
         <label>Whisper Server URL (leer = lokal)</label>
-        <input type="text" id="whisper-url" placeholder="http://beastix:8000">
+        <input type="text" id="whisper-url" placeholder="http://beastix:8080">
       </div>
       <div class="field">
         <label>Whisper Modell</label>