feat: Cloud-Fundament - Auth, Persistenz und Mandanten-Trennung

- SQLite-Store (app/store.py) hinter Store-Interface: Nutzer + Sessions persistent
- Bearer-Token-Auth (app/auth.py); Nutzerverwaltung via Admin-Key (POST /api/admin/users)
- GET /api/me, PUT /api/me/prefs (dauerhafte Nutzer-Praeferenzen)
- chat/speak/transcribe/sessions auth-geschuetzt; Mandanten-Trennung (fremde Session -> 403)
- Route-Aufloesung: Defaults < Profil < ENV < Nutzer-Prefs < Session < Request
- SessionManager (in-memory) durch Store ersetzt
- AUTH_ENABLED-Schalter (prod an, dev/Tests aus); DB_PATH/ADMIN_API_KEY
- Doku aktualisiert (README, BEDIENUNGSANLEITUNG, Architektur, deploy-env); data/ gitignored
- Tests: 29 gruen (Auth, Mandanten, Persistenz, Routing)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/config.py

@@ -120,6 +120,9 @@ class Settings(BaseSettings):
     local_llm_base_url: str = "http://127.0.0.1:11434/v1"
     local_llm_api_key: str = "dummy"
     local_llm_model: str = "llama3.1"
+    db_path: str = str(BASE_DIR / "data" / "voice-assistant.db")
+    admin_api_key: str = ""
+    auth_enabled: bool = True
     model_config = SettingsConfigDict(
         env_file=ENV_FILE, case_sensitive=False, extra="ignore"
     )