Restore album_for_id and track_for_id functions in metadata_plugins to
support data source-specific lookups. These functions accept both an ID
and data_source parameter, enabling plugins like mbsync and missing to
retrieve metadata from the correct source.
Update mbsync and missing plugins to use the restored functions with
explicit data_source parameters. Add data_source validation to prevent
lookups when the source is not specified.
Add get_metadata_source helper function to retrieve plugins by their
data_source name, cached for performance.
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