Migration 0002 added fsid column with DEFAULT 0 but never
populated existing rows. Now PopulateLegacyFSIDs() runs after 0002
to extract fsid from legacy options string or generate random.
Also: move generateRandomFSID to internal/system to avoid import
cycles. Migration 0004 drops the legacy options column.
Version: 0.3.1 -> 0.3.2
