Exercises the concurrent reader/writer scenario that the atomic swap
fix in app.py addresses. Uses a _SharedCache helper that mirrors the
module-level _token_cache global — both reader and writer access the
same .current reference, so the GIL-atomic swap is properly tested.
6 tests: swap correctness, concurrent readers (4 threads x 100 refreshes,
zero empty reads), app.state sync, multiple prefixes, empty DB, and
concurrent refresh from 4 threads.
