stats_links.py: pure functions returning a filter per stats category
(sessions for game/platform/month, games played, total/refunded/dropped/
unfinished/finished/finished-released/bought-and-finished/backlog purchases).
Each is verified to produce a queryset whose count equals the stat it links
from, on single-game data (the modeling norm).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RF5L4HtbcykTfY9YUYGds3
