Increase slow query threshold to 50ms

The 10ms threshold was too noisy - queries in the 10-20ms range are
actually performing well after the index optimizations. Setting the
threshold to 50ms will help identify truly problematic queries.

internal/database/slowquery.go

@@ -7,7 +7,7 @@ import (
 	"time"
 )
 
-const slowQueryThreshold = 10 * time.Millisecond
+const slowQueryThreshold = 50 * time.Millisecond
 
 // logSlowQuery logs queries that take longer than slowQueryThreshold
 func logSlowQuery(logger *slog.Logger, query string, start time.Time) {