fix(11-07): roll back the savepoint when a swallowed read failed

- beachcomber and lighthouse released their savepoint whenever the inner
  function reported no error; a Gate that counts a failed read as off,
  or a channel function or delete snapshot that swallows one, left the
  caller's Postgres transaction aborted (25P02) and failed the write
- a failed RELEASE now rolls back to the savepoint, as the READMEs promise
- beachcomber gets its testcontainers harness and sync tests
  (TestSyncGates, TestSyncAfterCommit, TestSyncDeleteAndSoftDelete,
  TestSyncFailuresNonFatal, TestServiceSetup); lighthouse gets
  TestBroadcastSwallowedReadFailure

modules/lighthouse/broadcast.go

@@ -469,7 +469,12 @@ func (s *Service) inSavepoint(db *gorm.DB, fn func(tx *gorm.DB) error) {
 		}
 		return
 	}
-	if inTx {
+	if inTx && tx.Exec("RELEASE SAVEPOINT "+savepoint).Error != nil {
+		// A statement inside fn failed although fn did not report it (a
+		// channel or payload function that treats a failed read as "no
+		// broadcast", or the delete snapshot's reload): the transaction is
+		// aborted and only a rollback to the savepoint keeps the write alive.
+		tx.RollbackTo(savepoint)
 		tx.Exec("RELEASE SAVEPOINT " + savepoint)
 	}
 }