Add preserveSecs to backup entries

We want to keep updates automatic and don't want to keep reminding
users that apps are getting updated etc (i.e beyong the weekly
digest). The reason to remind them is so they can check if the
app updated correctly. in some very corner cases, the app is not
really checked upon for a while and people forget about them until
they check them later. in such cases, it's too late to recover because
the backpus gets cleaned up. this preserve seconds fields, let's us
mark 'update' backups for preservation for 3 weeks.

migrations/schema.sql

@@ -141,6 +141,7 @@ CREATE TABLE IF NOT EXISTS backups(
     state VARCHAR(16) NOT NULL,
     manifestJson TEXT, /* to validate if the app can be installed in this version of box */
     format VARCHAR(16) DEFAULT "tgz",
+    preserveSecs INTEGER DEFAULT 0,
 
     PRIMARY KEY (id));