feat: agent-02-kiosk Phase 1 - NFC UID migration + session service

- Migration 0025: kiosk_nfc_uid column on users table with partial unique index per company
- User model: kiosk_nfc_uid field after personnel_number
- New service: kiosk_session_service.py (Redis-based 15min sessions)
- New core module: app/core/redis.py (sync Redis client with ping-test)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/core/redis.py

@@ -0,0 +1,28 @@
+"""Redis-Client für TimeMaster (sync, für Kiosk-Nonce-Cache und Sessions)."""
+from __future__ import annotations
+
+import logging
+from typing import Optional
+
+log = logging.getLogger(__name__)
+
+_redis_client = None
+
+
+def get_redis_client():
+    """Gibt den Redis-Client zurück oder None wenn nicht konfiguriert/erreichbar."""
+    global _redis_client
+    if _redis_client is not None:
+        return _redis_client
+    try:
+        import redis as redis_lib
+        from app.core.config import settings
+        url = getattr(settings, "redis_url", "redis://localhost:6379/0")
+        _redis_client = redis_lib.from_url(url, decode_responses=True, socket_connect_timeout=2)
+        # Verbindung testen
+        _redis_client.ping()
+        log.info("Redis-Verbindung hergestellt: %s", url)
+        return _redis_client
+    except Exception as exc:
+        log.warning("Redis nicht verfügbar: %s", exc)
+        return None