I have left various ReadingLists issues unaddressed as they don't matter much as long as the extension gets limited use, and working on them would block other work that blocks the apps from using reading lists. These should be fixed before the service gets serious load (so, Q3-ish at the latest).

• prevent adding/removing entries from deleted lists?
• use non-offset-based continuation where possible (improves performance and robustness)
• remove /setup, setup automatically on first write?
• better names for things? (esp. setup/teardown)
• pin down the semantics of the metadata fields (color, image, icon). And for that matter, the fields themselves. T180092
• rethink where/how to return list/entry order
• from T182053: getListsByDateUpdated has two dedicated indexes, but it could probably use the same indexes as getAllLists with some trickery
• from T182053: getListEntries filesorts when asked for entries of multiple lists; this could probably be avoided by sorting by list ID first.