fix: exiftool JSON output decoded with locale encoding instead of UTF-8

[hanhan761]

ExifTool always outputs JSON in UTF-8, but the exiftool_metadata() function decodes the output using locale.getpreferredencoding(). On systems where the locale encoding is not UTF-8 (e.g., Windows with Chinese locale uses cp936/GBK), this can cause a UnicodeDecodeError when exiftool returns metadata containing non-ASCII characters.

Reproduction:

1. Use Windows with Chinese locale (cp936)
2. Run exiftool on an image with non-ASCII metadata
3. exiftool_metadata() raises UnicodeDecodeError

Expected: JSON output decoded as UTF-8 (which is always the correct encoding for JSON per RFC 8259).