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)

.gitignore

@@ -0,0 +1,9 @@
+recordings/
+recorder.log
+config.ini
+__pycache__/
+*.pyc
+*.pyo
+.pytest_cache/
+.claude/
+nul