refactor: tighten LuaCATS annotations across modules

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`.

lua/pending/diff.lua

@@ -7,11 +7,11 @@ local parse = require('pending.parse')
 ---@field id? integer
 ---@field description? string
 ---@field priority? integer
----@field status? string
+---@field status? pending.TaskStatus
 ---@field category? string
 ---@field due? string
 ---@field rec? string
----@field rec_mode? string
+---@field rec_mode? pending.RecurMode
 ---@field forge_ref? pending.ForgeRef
 ---@field lnum integer