Metadata: Skip brute-force search for standard file formats if no Exif headers were found

[lastzero]

As a user with many media files, I would like to skip brute-force search for Exif data when no standard file headers were found so that indexing of files without metadata is faster.

Users can turn the old behavior back on by setting PHOTOPRISM_EXIF_BRUTEFORCE: "true" in their docker-compose.yml config or with the --exif-bruteforce command-line flag.