chore(docker): explanations in .env.example, whisper as commented block

- docker-compose.yml only maps variables; every variable is explained in
  .env.example (German, with defaults, units and choices).
- The whisper service is a commented block to switch on instead of a
  compose profile.
- TZ, TRUSTED_PROXIES, LOGIN_PROTECTION, DEMO_RESET_MINUTES and
  LEGAL_*_CONTENT are now passed through (they had no effect from .env).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -123,6 +123,8 @@ curl -fsSL https://git.dsit.me/sandrodach/MindSift/raw/branch/dev/.env.example -
 docker compose up -d
 ```
 
+Alle Einstellungen stehen mit Erklärung in `.env`. Optional kommt eine eigene Spracherkennung dazu: Block `whisper` in `docker-compose.yml` einkommentieren und `STT_URL=http://whisper:8000` setzen (Anleitung in der Datei).
+
 Minimal:
 
 ```yaml