Alerting: Fix saving of screenshots uploaded with a signed url (#53933)

The URL of screenshots uploaded to external image storages can be optionally signed, resulting in a long string (800+ chars).

pkg/services/sqlstore/migrations/ualert/tables.go

@@ -396,6 +396,11 @@ func AddAlertImageMigrations(mg *migrator.Migrator) {
 			{Cols: []string{"token"}, Type: migrator.UniqueIndex},
 		},
 	}
+
 	mg.AddMigration("create alert_image table", migrator.NewAddTableMigration(imageTable))
 	mg.AddMigration("add unique index on token to alert_image table", migrator.NewAddIndexMigration(imageTable, imageTable.Indices[0]))
+
+	mg.AddMigration("support longer URLs in alert_image table", migrator.NewRawSQLMigration("").
+		Postgres("ALTER TABLE alert_image ALTER COLUMN url TYPE VARCHAR(2048);").
+		Mysql("ALTER TABLE alert_image MODIFY url VARCHAR(2048) NOT NULL;"))
 }