refactor: use Params struct for imageprocessor constructor

Rename NewImageProcessor(maxInputBytes) to New(Params{}) with a Params
struct containing MaxInputBytes. Zero-value Params{} uses sensible
defaults (DefaultMaxInputBytes). All callers updated.

Addresses review feedback on PR #37.

internal/imgcache/processor.go

@@ -44,11 +44,20 @@ type ImageProcessor struct {
 	maxInputBytes int64
 }
 
-// NewImageProcessor creates a new image processor with the given maximum input
-// size in bytes. If maxInputBytes is <= 0, DefaultMaxInputBytes is used.
-func NewImageProcessor(maxInputBytes int64) *ImageProcessor {
+// Params holds configuration for creating an ImageProcessor.
+// Zero values use sensible defaults (MaxInputBytes defaults to DefaultMaxInputBytes).
+type Params struct {
+	// MaxInputBytes is the maximum allowed input size in bytes.
+	// If <= 0, DefaultMaxInputBytes is used.
+	MaxInputBytes int64
+}
+
+// New creates a new image processor with the given parameters.
+// A zero-value Params{} uses sensible defaults.
+func New(params Params) *ImageProcessor {
 	initVips()
 
+	maxInputBytes := params.MaxInputBytes
 	if maxInputBytes <= 0 {
 		maxInputBytes = DefaultMaxInputBytes
 	}