fix(memory): use staging folder to avoid keeping everything in memory

Co-authored-by: Belphemur <197810+Belphemur@users.noreply.github.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

Fixes #203

internal/manga/chapter.go

@@ -1,6 +1,10 @@
 package manga
 
-import "time"
+import (
+	"fmt"
+	"os"
+	"time"
+)
 
 type Chapter struct {
 	// FilePath is the path to the chapter's directory.
@@ -13,6 +17,11 @@ type Chapter struct {
 	IsConverted bool
 	// ConvertedTime is a pointer to a time.Time object that indicates when the chapter was converted. Nil mean not converted.
 	ConvertedTime time.Time
+	// TempDir, when non-empty, is a staging temp folder holding converted
+	// page contents on disk (see Page.TempFilePath) rather than fully in
+	// memory. It should be removed via Cleanup once the chapter has been
+	// written out (or is no longer needed).
+	TempDir string
 }
 
 // SetConverted sets the IsConverted field to true and sets the ConvertedTime field to the current time.
@@ -20,3 +29,16 @@ func (chapter *Chapter) SetConverted() {
 	chapter.IsConverted = true
 	chapter.ConvertedTime = time.Now()
 }
+
+// Cleanup removes the chapter's staging temp folder (if any), releasing any
+// page contents that were staged to disk during conversion.
+func (chapter *Chapter) Cleanup() error {
+	if chapter.TempDir == "" {
+		return nil
+	}
+	if err := os.RemoveAll(chapter.TempDir); err != nil {
+		return fmt.Errorf("failed to remove staging temp folder: %w", err)
+	}
+	chapter.TempDir = ""
+	return nil
+}