db::apply_migrations runs ALTER lists and ignores duplicate-column
errors (turn_stats' pattern); approvals, operator_questions, broker
reminders, and scheduled_prompts drop their hand-rolled
pragma_table_info guards. broker's acked_at migration stays bespoke —
its backfill must only run when the column was just created
one db::open owns the parent-dir + connection + busy_timeout dance for
every host-side store (broker/approvals/questions/schedules/power in
broker.sqlite, build_logs, audit_log); schema + migrations stay per
store. same-file connections now wait out concurrent writers instead
of risking SQLITE_BUSY.