Problem: type annotations repeated inline unions with no aliases,
used `table<string, any>` where structured types exist, and had
loose `string` where union types should be used.
Solution: add `pending.TaskStatus`, `pending.RecurMode`,
`pending.TaskExtra`, `pending.ForgeType`, `pending.ForgeState`,
`pending.ForgeAuthStatus` aliases and `pending.SyncBackend`
interface. Replace inline unions and loose types with the new
aliases in `store.lua`, `forge.lua`, `config.lua`, `diff.lua`,
`views.lua`, `parse.lua`, `init.lua`, and `oauth.lua`.
Barrett Ruth
4710e6197f