feat(broker): migrations-based install, roles, RLS, and job dependency groups

Replace the ad-hoc tables/procedures install layout with versioned,
ordered SQL migrations tracked in broker_schema_migrations. Add
optional least-privilege role provisioning (--with-roles), multi-tenant
row-level security, lease-based job claiming with stale-lease recovery,
and job dependencies -- both by job id and by fan-in job group. Add
Docker/Compose support for running the broker and its test suite.

pkg/broker/broker.go

@@ -100,6 +100,7 @@ func (b *Broker) stopInstances() {
 		wg.Add(1)
 		go func(inst *DatabaseInstance) {
 			defer wg.Done()
+			defer adapter.RecoverAndLog(b.logger, "stop-instance-"+inst.DatabaseName)
 			if err := inst.Stop(); err != nil {
 				b.logger.Error("failed to stop instance", "name", inst.DatabaseName, "error", err)
 			}