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/beachcomber/sync.go

@@ -282,7 +282,13 @@ func inSavepoint(db *gorm.DB, fn func(tx *gorm.DB) error) error {
 		db.RollbackTo(syncSavepoint)
 		return err
 	}
-	db.Exec("RELEASE SAVEPOINT " + syncSavepoint)
+	if db.Exec("RELEASE SAVEPOINT "+syncSavepoint).Error != nil {
+		// A statement inside fn failed although fn did not report it (a
+		// Gate counts a failed read as off): the transaction is aborted
+		// and only a rollback to the savepoint makes it usable again.
+		db.RollbackTo(syncSavepoint)
+		db.Exec("RELEASE SAVEPOINT " + syncSavepoint)
+	}
 	return nil
 }