tasks: add completed flag

in some cases, the tasks are setting percent to 100 and crashing later

migrations/schema.sql

@@ -251,6 +251,7 @@ CREATE TABLE IF NOT EXISTS tasks(
     type VARCHAR(32) NOT NULL,
     argsJson TEXT,
     pending BOOLEAN DEFAULT true,
+    completed BOOLEAN DEFAULT false,
     percent INTEGER DEFAULT 0,
     message TEXT,
     errorJson TEXT,