fix(11-07): enqueue broadcast jobs before GORM commits a single write

- lighthouse:after_create/update/delete also declare
  Before(gorm:commit_or_rollback_transaction); an After-only anchor put
  them past GORM's own commit, so a plain gdb.Create enqueued its
  broadcast job after the commit on the pool (deferred from 11-05)
- lighthouse gets the testcontainers Postgres harness and TestBroadcastTx
  (commit publishes once, rollback nothing, single-statement write
  enqueues on its own transaction, failed write enqueues nothing)

modules/lighthouse/broadcast.go

@@ -229,20 +229,26 @@ type defaultPayload struct {
 	TTL       int       `json:"ttl"`
 }
 
+// commitCallback is GORM's commit of a transaction it opened itself.
+const commitCallback = "gorm:commit_or_rollback_transaction"
+
 // installCallbacks registers the broadcast callbacks on gdb, replacing
 // earlier ones so a handle shared by several apps broadcasts through the
-// most recent service.
+// most recent service. The after-write callbacks run after the model's own
+// after hook and before GORM commits a single-statement write, so the job
+// is enqueued on the write's transaction: GORM appends a callback that
+// names only an After anchor to the end of the chain, past the commit.
 func (s *Service) installCallbacks(gdb *gorm.DB) error {
 	cb := gdb.Callback()
 	if cb.Create().Get(CallbackAfterCreate) == nil {
-		if err := cb.Create().After("gorm:after_create").Register(CallbackAfterCreate, s.afterCreate); err != nil {
+		if err := cb.Create().After("gorm:after_create").Before(commitCallback).Register(CallbackAfterCreate, s.afterCreate); err != nil {
 			return err
 		}
 	} else if err := cb.Create().Replace(CallbackAfterCreate, s.afterCreate); err != nil {
 		return err
 	}
 	if cb.Update().Get(CallbackAfterUpdate) == nil {
-		if err := cb.Update().After("gorm:after_update").Register(CallbackAfterUpdate, s.afterUpdate); err != nil {
+		if err := cb.Update().After("gorm:after_update").Before(commitCallback).Register(CallbackAfterUpdate, s.afterUpdate); err != nil {
 			return err
 		}
 	} else if err := cb.Update().Replace(CallbackAfterUpdate, s.afterUpdate); err != nil {
@@ -256,7 +262,7 @@ func (s *Service) installCallbacks(gdb *gorm.DB) error {
 		return err
 	}
 	if cb.Delete().Get(CallbackAfterDelete) == nil {
-		if err := cb.Delete().After("gorm:after_delete").Register(CallbackAfterDelete, s.afterDelete); err != nil {
+		if err := cb.Delete().After("gorm:after_delete").Before(commitCallback).Register(CallbackAfterDelete, s.afterDelete); err != nil {
 			return err
 		}
 	} else if err := cb.Delete().Replace(CallbackAfterDelete, s.afterDelete); err != nil {