fix: set image format before calling write() in ImageMagick

The correct API pattern is:
1. Set image.format = outputFormatEnum first
2. Then call image.write(callback) with the format already set

This ensures the image is written in the correct format with proper
headers (RIFF for WebP, etc.)

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

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

lib/converters/imagemagickService.ts

@@ -39,6 +39,9 @@ export async function convertWithImageMagick(
     let result: Uint8Array | undefined;
 
     await ImageMagick.read(inputData, (image) => {
+      // Set output format
+      image.format = outputFormatEnum;
+
       // Apply quality setting if specified
       if (options.imageQuality !== undefined) {
         image.quality = options.imageQuality;
@@ -65,8 +68,10 @@ export async function convertWithImageMagick(
 
       if (onProgress) onProgress(70);
 
-      // Write the image data with the specified format
-      result = image.write(outputFormatEnum);
+      // Write the image data in the specified format
+      image.write((data) => {
+        result = data;
+      });
 
       if (onProgress) onProgress(90);
     });