fix(volume): preserve DB UUID on cache refresh

When a tracked volume gets updated in the cache (mount status change,
space change, etc.), line 741 was overwriting the ID with existing.id
which could be an ephemeral Uuid::new_v4() if the cache was populated
before DB metadata was merged (startup race condition). This defeats
the stable UUID fix from 4f387eede.

Now prefers the DB UUID from tracked_volumes_map when available,
falling back to existing.id only for untracked volumes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>