fix(db): add LIMIT to pair frequency query to prevent OOMKill in acb-index-builder

The fetchOpenPredictions function had an unbounded query building a pair
frequency map for rivalry detection. With thousands of bots and matches,
this could return tens of thousands of rows and cause OOMKill.

- Add ORDER BY COUNT(*) DESC to prioritize most common pairings
- Add LIMIT 1000 - sufficient to detect rivalries (pairs with >= 3 matches)

This fixes the 45-day CrashLoopBackOff with 4700+ restarts.

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

cmd/acb-index-builder/db.go

@@ -912,6 +912,8 @@ func fetchOpenPredictions(ctx context.Context, db *sql.DB) ([]OpenPredictionMatc
 	}
 
 	// Build pair frequency map for rivalry detection (count completed h2h matches)
+	// LIMIT to most common pairings to prevent OOMKill - we only need enough
+	// to detect rivalries (typically < 100 pairs have >= 3 matches)
 	pairFrequency := make(map[string]int)
 	freqRows, err := db.QueryContext(ctx, `
 		SELECT mp1.bot_id, mp2.bot_id, COUNT(*)
@@ -920,6 +922,8 @@ func fetchOpenPredictions(ctx context.Context, db *sql.DB) ([]OpenPredictionMatc
 		JOIN match_participants mp2 ON m.match_id = mp2.match_id AND mp2.player_slot = 1
 		WHERE m.status = 'completed'
 		GROUP BY mp1.bot_id, mp2.bot_id
+		ORDER BY COUNT(*) DESC
+		LIMIT 1000
 	`)
 	if err != nil {
 		return nil, fmt.Errorf("query pair frequency: %w", err)