feat: register ebook converter in the app so the API serves the new family

Also fixes a real bug found via the fb2 API test: file-type sniffs a
real fb2 file's XML declaration as generic "xml", not "fb2" and not
undetected, so it never reached the undetectable-format fallback added
in the previous commit. Added an fb2->xml alias in normalizeFormat,
same pattern as the existing azw3->mobi one.

src/mime.js

@@ -66,6 +66,7 @@ function normalizeFormat(format) {
   if (format === 'jpg') return 'jpeg';
   if (format === 'heif') return 'heic';
   if (format === 'azw3') return 'mobi';
+  if (format === 'fb2') return 'xml';
   return format;
 }