fix: use correct ImageMagick write API to produce valid output format

Changed from image.write((data) => data) callback pattern to
image.write(outputFormatEnum) which correctly writes the image data
in the specified format. This should now produce valid WebP files
with proper RIFF headers (52 19 46 46 magic bytes).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

lib/converters/imagemagickService.ts

@@ -39,9 +39,6 @@ export async function convertWithImageMagick(
     let result: Uint8Array | undefined;
 
     await ImageMagick.read(inputData, (image) => {
-      // Set the output format first
-      image.format = outputFormatEnum;
-
       // Apply quality setting if specified
       if (options.imageQuality !== undefined) {
         image.quality = options.imageQuality;
@@ -68,8 +65,8 @@ export async function convertWithImageMagick(
 
       if (onProgress) onProgress(70);
 
-      // Write the image data
-      result = image.write((data) => data);
+      // Write the image data with the specified format
+      result = image.write(outputFormatEnum);
 
       if (onProgress) onProgress(90);
     });