fix: correct Stats() to scan only hit/miss counts, compute HitRate properly

Stats() was scanning 5 SQL columns (hit_count, miss_count,
upstream_fetch_count, upstream_fetch_bytes, transform_count) into
mismatched struct fields, causing HitRate to contain the integer
transform_count instead of a 0.0-1.0 ratio.

Simplify the query to only fetch hit_count and miss_count, then
compute TotalItems, TotalSizeBytes, and HitRate correctly.

Fixes #4

internal/imgcache/cache.go

@@ -297,19 +297,21 @@ func (c *Cache) CleanExpired(ctx context.Context) error {
 func (c *Cache) Stats(ctx context.Context) (*CacheStats, error) {
 	var stats CacheStats
 
+	// Fetch hit/miss counts from the stats table
 	err := c.db.QueryRowContext(ctx, `
-		SELECT hit_count, miss_count, upstream_fetch_count, upstream_fetch_bytes, transform_count
+		SELECT hit_count, miss_count
 		FROM cache_stats WHERE id = 1
-	`).Scan(&stats.HitCount, &stats.MissCount, &stats.TotalItems, &stats.TotalSizeBytes, &stats.HitRate)
+	`).Scan(&stats.HitCount, &stats.MissCount)
 
 	if err != nil && !errors.Is(err, sql.ErrNoRows) {
 		return nil, fmt.Errorf("failed to get cache stats: %w", err)
 	}
 
-	// Get actual counts
+	// Get actual item count and total size from content tables
 	_ = c.db.QueryRowContext(ctx, `SELECT COUNT(*) FROM request_cache`).Scan(&stats.TotalItems)
 	_ = c.db.QueryRowContext(ctx, `SELECT COALESCE(SUM(size_bytes), 0) FROM output_content`).Scan(&stats.TotalSizeBytes)
 
+	// Compute hit rate as a ratio
 	if stats.HitCount+stats.MissCount > 0 {
 		stats.HitRate = float64(stats.HitCount) / float64(stats.HitCount+stats.MissCount)
 	}