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/migrations/versions/0025_kiosk_nfc_uid.py

@@ -0,0 +1,36 @@
+"""Add kiosk_nfc_uid to users table
+
+Revision ID: 0025
+Revises: 0024
+Create Date: 2026-05-24
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "0025"
+down_revision = "0024"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # NFC-UID für Kiosk-Login (optional, eindeutig pro Firma über partial unique index)
+    op.add_column("users", sa.Column("kiosk_nfc_uid", sa.String(64), nullable=True))
+    op.create_index(
+        "ix_users_kiosk_nfc_uid",
+        "users",
+        ["kiosk_nfc_uid"],
+        unique=False,  # Uniqueness nur innerhalb company → partial index
+    )
+    # Partial unique index: NFC-UID eindeutig innerhalb einer Firma
+    op.execute("""
+        CREATE UNIQUE INDEX IF NOT EXISTS uq_users_nfc_uid_per_company
+        ON users (company_id, kiosk_nfc_uid)
+        WHERE kiosk_nfc_uid IS NOT NULL
+    """)
+
+
+def downgrade() -> None:
+    op.execute("DROP INDEX IF EXISTS uq_users_nfc_uid_per_company")
+    op.drop_index("ix_users_kiosk_nfc_uid", table_name="users")
+    op.drop_column("users", "kiosk_nfc_uid")