Count a late hit, and report no totals on a disabled cache (closes #56)

A hit is now counted with the request context detached from its
cancellation, as the miss already is, so a hit served after the client
left or the request timed out still moves the hit count. On a disabled
disk cache, Stats reports no items and no size, whatever rows an earlier
run left in the database; the size already followed that rule.

Model: opus-5-5

internal/imgcache/cache.go

@@ -419,18 +419,21 @@ func (c *Cache) Stats(ctx context.Context) (*CacheStats, error) {
 		return nil, fmt.Errorf("failed to get cache stats: %w", err)
 	}
 
-	// Count and size the cached source images and processed variants
-	err = c.db.QueryRowContext(ctx, `
-		SELECT (SELECT COUNT(*) FROM source_content)
-		     + (SELECT COUNT(*) FROM variant_content)
-	`).Scan(&stats.TotalItems)
-	if err != nil {
-		c.log.Warn("failed to count cache items for stats", "error", err)
-	}
+	// Count and size the cached source images and processed variants. A
+	// disabled cache holds none, whatever rows an earlier run left.
+	if !c.disabled {
+		err = c.db.QueryRowContext(ctx, `
+			SELECT (SELECT COUNT(*) FROM source_content)
+			     + (SELECT COUNT(*) FROM variant_content)
+		`).Scan(&stats.TotalItems)
+		if err != nil {
+			c.log.Warn("failed to count cache items for stats", "error", err)
+		}
 
-	stats.TotalSizeBytes, err = c.UsageBytes(ctx)
-	if err != nil {
-		c.log.Warn("failed to sum cache size for stats", "error", err)
+		stats.TotalSizeBytes, err = c.UsageBytes(ctx)
+		if err != nil {
+			c.log.Warn("failed to sum cache size for stats", "error", err)
+		}
 	}
 
 	// Compute hit rate as a ratio