Add detailed logging for image conversions on cache miss

Log includes:
- file path
- input/output format
- input/output size in bytes
- input/output dimensions
- size ratio (percentage)

Also adds InputWidth, InputHeight, InputFormat to ProcessResult

internal/imgcache/imgcache.go

@@ -187,6 +187,12 @@ type ProcessResult struct {
 	Width int
 	// Height is the output image height
 	Height int
+	// InputWidth is the original image width before processing
+	InputWidth int
+	// InputHeight is the original image height before processing
+	InputHeight int
+	// InputFormat is the detected input format (e.g., "jpeg", "png")
+	InputFormat string
 }
 
 // Storage handles persistent storage of cached content