get_user_client_basic_auth built a fresh RefreshTokenStorage and ran
storage.initialize() — a full Alembic `upgrade` in a worker thread — on every
call. Once the /api/v1 search endpoints (unified_search, vector_search) and the
chunk-context endpoint were wired to use it, concurrent requests ran concurrent
Alembic upgrades, which race on Alembic's non-thread-safe module-global
EnvironmentContext proxy and intermittently raise `KeyError: 'script'` →
HTTP 500 (seen on multi-user-basic/nc31; nc32 got lucky).
Cache one process-wide, already-initialized storage instance behind a lazily
created anyio.Lock so the one-time migration runs exactly once and never races.
Callers passing an explicit `storage` are unaffected. Also removes a redundant
per-request migration from the search hot path.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
