backups: add per site enabledForUpdates flag

previously, we had a singleton 'main' flag to indicate a site can
be used for updates. with this new approach, we can get rid of the
'primary' concept. each site can be used for updates or not.

migrations/schema.sql

@@ -316,7 +316,7 @@ CREATE TABLE IF NOT EXISTS backupSites(
     format VARCHAR(16) NOT NULL,
     schedule VARCHAR(128),
     integrityKeyPairJson TEXT, // { publicKey, privateKey }
-    main BOOLEAN DEFAULT false, // 'primary' and 'default' are SQL keywords
+    enabledForUpdates BOOLEAN DEFAULT false,
     creationTime TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
     ts TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,