Add index on playback_log.playtime

data/sql/dbmigrate-29_to_30.sql

@@ -0,0 +1,6 @@
+-- Script to migate from db version 29 to 30.
+
+-- Add the following index to speed up all Operations that deal with a specific time period of plays
+CREATE INDEX playback_log_playtime ON playback_log(playtime);
+
+UPDATE settings SET v = '30' WHERE k == 'schema_version';