Bug: stats.lua cannot handle deep nesting

stats.lua tries to calculate message threading.

However if too many messages reference each other, the code crashes with an error:

site/api/stats.lua:755: Cannot serialise, excessive nesting (501)

This prevents the list data from being displayed.

The code should fail gracefully if the nesting depth is too great.

Start in site/api/stats.lua around line 755 and trace how deeply nested message threading reaches serialization. Reproduce the excessive-nesting case, then verify that the code fails gracefully and the list data is no longer prevented from being displayed.

Written by the indexing model from the issue text.