perf(migrations): index map_events_daily by day

Its key starts with the zone, so the economy compaction read the whole table twice for every
day it merged into a month. With 900k old rows to merge on MariaDB the task went from 107 s to
14 s. The index holds only the day and the key, so the game's upsert that adds to a row's
events never updates it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

migrations/dlu/sqlite/58_map_events_day_index.sql

@@ -0,0 +1,2 @@
+/* map_events_daily by day, for the economy compaction and the reports over every zone: see the MySQL migration */
+CREATE INDEX IF NOT EXISTS map_events_daily_day ON map_events_daily (day);