feat(store): keep each list as it was last read

[noctuum]

Migration 3 adds two tables. backend_ids links a show to an entry of a backend by the number the service gives the work, and a show is at most one entry of a backend. list_entries holds what each list held when it was last read.

• Store::file takes one backend's list in one transaction. It files and links a show for each entry with no link, gives a linked show the entry's title, and replaces that backend's rows of list_entries. A link stays when its entry leaves the list, so an entry that comes back is the show it was.
• Store::listed answers every show of a backend's list with its Entry. A row whose show is gone is refused as Error::Showless, and a row that cannot be read as Error::Entry, each naming the entry.
• benshi_core::list holds Entry, EntryId and Format. Status and Rating move there from benshi_store::queue.

cargo test --workspace passes, 844 tests.