mark_failed never sets status to failed in the downloader

[haydenk]

In crates/downloader/src/main.rs, mark_failed() only writes to error_message and never changes the status column. Failed downloads stay in 'discovered' indefinitely — the 'failed' status enum value is dead code for the downloader, and there's no way to distinguish "never tried" from "tried and failed."

Fix: Set status = 'failed' in the mark_failed UPDATE query alongside the error_message write.