Updated mock AAC transcoding command to use the new default (ipod with
fragmented MP4) matching the migration, ensuring tests exercise the same
buildDynamicArgs code path as production. Improved archiver test mock to
match on the whole StreamRequest struct instead of decomposing fields,
making it resilient to future field additions. Added named constants for
JWT claim keys in the transcode token and wrapped ParseTranscodeParams
errors with ErrTokenInvalid for consistency. Documented the IsLossless
BitDepth fallback heuristic as temporary until Codec column is populated.
Signed-off-by: Deluan <deluan@navidrome.org>
