appview/db: fix timeline queries to work with repoDids
when selecting fork sources, we were selecting by at_uri, which was
causing the queries to sliently fail. switching to repoDids here fixes
fork sources, and as a result, fixes the templates to render the right
repo card text: `<user> forked <repo> to <repo>`.
Signed-off-by: oppiliappan <me@oppi.li>