Add two service-level integration tests to JourneyItemIntegrationTest:
- append_persists_item_at_position_10: verifies that the first append to an
empty journey creates an item at position 10 in the DB.
- reorder_swaps_positions_atomically: appends two items then reorders them,
asserting the DB reflects the new position assignment.
Both tests use the SecurityContextHolder authentication pattern from
GeschichteServiceIntegrationTest and mock S3Client to avoid MinIO connections.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>