perf: skip achievement metric recounts for live player count updates

[wescopeland]

Addresses RAWEB-1QM.

When a player's unlock count for a game becomes non-zero, we do two different jobs worth of work:

1. We update the game's player count. "The game now has 80,001 players instead of 80,000."
2. We recount the stats for every achievement on the game's set from scratch. "For achievement X, count all unlocks. For achievement Y, count all unlocks. For achievement Z... etc".

That second step is the slow player_achievements GROUP BY achievement_id query from RAWEB-1QM.

This PR makes a minor adjustment: for these player count driven cascades, we'll no longer recount every achievement's unlock totals. Instead, we'll use the unlock totals already cached on each achievement's row and just recompute the unlock percentage, unlock hardcore percentage, and weighted points.

In other words, this PR introduces a "fast track" path for when only the player count changes. The "slow track" path (current behavior) is still present for:

• achievements promoted / demoted
• achievement transfers
• resets
• players getting untracked / retracked