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/models/user.py

@@ -59,6 +59,9 @@ class User(Base):
     # Personalnummer (numerisch, eindeutig pro Firma; bleibt nach Deaktivierung reserviert)
     personnel_number: Mapped[str | None] = mapped_column(String(50))
 
+    # NFC-UID für Kiosk-Login (optional, eindeutig pro Firma)
+    kiosk_nfc_uid: Mapped[str | None] = mapped_column(String(64), nullable=True, index=True)
+
     # TOTP / 2FA
     totp_secret: Mapped[str | None] = mapped_column(String(64))
     totp_enabled: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)