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/api/transcribe.py

@@ -1,6 +1,8 @@
-from fastapi import APIRouter, File, Form, HTTPException, Query, UploadFile
+from fastapi import APIRouter, Depends, File, Form, HTTPException, Query, UploadFile
 
 from app.errors import RoutingError
+from app.auth import require_user
+from app.store import User, SessionOwnershipError
 from app.dependencies import (
     resolve_route,
     build_orchestrator,
@@ -20,17 +22,20 @@ async def transcribe(
         default=None,
         description="Optional session id to apply a stored route",
     ),
+    user: User = Depends(require_user),
 ):
     overrides = {
         "input_endpoint": input_endpoint,
         "language": language,
         "stt_provider": stt_provider,
     }
-    route = resolve_route(session_id, overrides)
 
     try:
+        route = resolve_route(user, session_id, overrides)
         orchestrator = build_orchestrator(route)
         source = await resolve_input_endpoint(route)
+    except SessionOwnershipError as exc:
+        raise HTTPException(status_code=403, detail=str(exc))
     except RoutingError as exc:
         raise HTTPException(status_code=422, detail=str(exc))