Inlines the body of `library_backfill_downloads` from
crate::library::commands::* directly into commands_v2/legacy_compat.rs,
mapping the legacy String error type to RuntimeError::Internal at every
return path. Adjusts the v2 return type to refer to
crate::library::BackfillReport (the canonical re-export from
qbz-library) instead of the deleted crate::library::commands path.
Deletes the legacy #[tauri::command] definition in
library/commands.rs. MetadataExtractor and BackfillReport are reused
unchanged from their crate::library re-exports.
No frontend invoke changes required.
Step 6 of the legacy cleanup plan, group E (backfill downloads).
cc
vicrodh
99729d3888