Fixes to support proper test coverage:
- db_jobs.go: UpdateJobStatus now checks RowsAffected and returns error
for nonexistent jobs instead of silently succeeding
- db_audit.go: GetOldestAuditLogDate uses sql.NullString to parse SQLite
datetime strings in YYYY-MM-DD HH:MM:SS format with RFC3339 fallback
- db_experiments.go: ListTasksForExperiment uses sql.NullString for
nullable worker_id and error fields to prevent scan errors
- db_connect.go: DB struct adds isClosed state tracking with mutex;
Close() now returns error on double close to match test expectations
