- store: guard UsageEventStore.shared() with a class-level anyio.Lock so
two concurrent cold-start callers don't both build (and one silently
overwrite) the cached instance — mirrors get_shared_storage(). Document
that tests should construct the store directly to avoid singleton leak.
- migration: rename 20260610 -> 20260607 and fix Create Date to today so
`alembic history` isn't future-dated (revision id 007 / down_revision
006 unchanged; single head verified).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>