Show downloads even without releases rows

Add synthetic release groups in getEntryById so downloads
are displayed even when there are no matching rows in
`releases` for a given entry. Group by `release_seq`,
attach downloads, and sort groups for stable order.

This fixes cases like /zxdb/entries/1 where `downloads`
exist for the entry but `releases` is empty, resulting in
no downloads shown in the UI.

Signed-off-by: Junie@devbox