- Separate updated_at into a meta sidecar file (msteams_conversations_meta.json)
to keep backward compatibility with legacy data that never had updated_at.
On first upgrade, legacy refs are kept alive by initializing updated_at to now
instead of purging them immediately.
- Add cross-process locking via fcntl (with Windows fallback) to prevent
concurrent writes from different gateway processes overwriting each other.
- Add ref_touch_interval_s config (default 300s) to throttle how often
successful sends refresh updated_at, preventing unnecessary I/O.
- Touch active refs on send success to prevent them from expiring while in use.
- Add _safe_float and _normalize_ref_record for robust schema migration.
- All refs operations now use threading.RLock within a process.
