feat: Tageskontingent und Notfall-Eskalation (#6)

- Quota (app/quota.py): Anfragen pro Nutzer/Tag (usage-Tabelle), DAILY_REQUEST_LIMIT,
  pro Nutzer via prefs uebersteuerbar; 429 (REST) bzw. error-Event (WS); Metrik
- Notfall (app/safety/emergency.py): heuristische Erkennung (de/en); Log im Store
  (emergency_events) + optionaler Webhook (best-effort) + X-Emergency/emergency-Event;
  Metrik emergency_total; Notfaelle umgehen das Kontingent
- verdrahtet in chat/speak/transcribe + WS-Turns
- Tests: 64 gruen (+6); Doku aktualisiert (README, BEDIENUNGSANLEITUNG, Architektur, .env.example)

Hinweis: Notfall-Erkennung ist eine Heuristik (kein Lebensretter); erkannte Texte
sind sensibel -> DSGVO beachten.

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

app/api/speak.py

@@ -11,7 +11,9 @@ from app.dependencies import (
     resolve_route,
     build_orchestrator,
     resolve_output_endpoint,
+    get_store,
 )
+from app.quota import enforce_quota, record_usage, QuotaExceededError
 from app.schemas import SpeakRequest
 
 router = APIRouter()
@@ -42,6 +44,12 @@ async def speak(
     except RoutingError as exc:
         raise HTTPException(status_code=422, detail=str(exc))
 
+    store = get_store()
+    try:
+        enforce_quota(user, store)
+    except QuotaExceededError as exc:
+        raise HTTPException(status_code=429, detail=str(exc))
+
     try:
         audio = await orchestrator.speak_only(
             payload.text,
@@ -49,6 +57,7 @@ async def speak(
             language=route.language,
             output=output,
         )
+        record_usage(user, store, len(payload.text))
 
         headers = {
             "Content-Language": route.language,