✨ Reduce the file-change snapshot taking ratio.

Until now, a file `data` snapshot was persisted on every file_change
row. That causes a lot of IO load and increase disk usage without
a real benefit.

This commit reduces the snapshot generation; now the snapshot
is persisted every 20 update-file or when a file is not touched
in 3 hours or more.

backend/src/app/migrations/sql/0060-mod-file-change-table.sql

@@ -0,0 +1,2 @@
+ALTER TABLE file_change
+ALTER COLUMN data DROP NOT NULL;