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/parse.lua

@@ -4,7 +4,7 @@ local config = require('pending.config')
 ---@field due? string
 ---@field cat? string
 ---@field rec? string
----@field rec_mode? 'scheduled'|'completion'
+---@field rec_mode? pending.RecurMode
 ---@field priority? integer
 
 ---@class pending.parse