Add Docker support, fix stale docs, translate UI to English

- Dockerfile + docker-compose.yml: two services (recorder + web) sharing
  ./recordings bind mount; recorder maps /dev/snd for ALSA soundcard access
- requirements.txt: requests, numpy, soundfile
- .dockerignore, updated .gitignore (add __pycache__, .pytest_cache)
- isr.py: add SIGTERM handler for clean Docker shutdown; fix stale error
  message that referenced removed PulseAudio/PipeWire/PortAudio backends
- web.py: translate all German UI strings to English
- config.example.ini: remove PipeWire/PulseAudio/PortAudio backend refs,
  simplify soundcard tips to ALSA only
- README.md: full rewrite as user guide (quick start, config reference,
  Docker notes, how it works)
- CLAUDE.md: update architecture section to reflect ALSA-only backend
- Delete changelog.txt and guide.md (internal session notes)

Dockerfile

@@ -0,0 +1,16 @@
+FROM python:3.12-slim
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    alsa-utils \
+  && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /app
+
+COPY requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
+
+COPY isr.py web.py ./
+
+RUN mkdir -p /recordings
+
+CMD ["python", "isr.py", "/app/config.ini"]