users.lastSignInAt is never written - the adoption instrument is accidentally-always-zero

users.lastSignInAt is NULL for all 157 terra rows; terra src/db has zero references to the column. Found 2026-08-17 by coder-terra-cc while investigating TERRA-17's nav-visibility complaint (see that WI's event log) - it is the obvious field anyone queries to answer 'is anybody using this', and it currently answers 'nobody' for a reason unrelated to truth (nobody writes it), which will stay true after real adoption starts. Shared users table: needs db-terra-cc + enamel coordination on the writer, not a coder-only patch. Not the same defect as TERRA-43 (probe-row/digest-signature contamination) - do not fold in there.