Inlines the bodies of `discogs_has_credentials`,
`discogs_search_artwork`, and `discogs_download_artwork` from
crate::library::commands::* directly into commands_v2/legacy_compat.rs.
The discogs delegates were single-line bodies (or trivial proxy logic);
DiscogsClient and library::get_artwork_cache_dir are reused as-is.
The String error type from the underlying DiscogsClient methods is
mapped to RuntimeError::Internal at the v2 boundary, matching the
existing v2 contracts.
Deletes the legacy #[tauri::command] definitions from
library/commands.rs. No frontend invoke changes required.
Step 6 of the legacy cleanup plan, group B (discogs).
cc
vicrodh
40b8ec90a0