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/init.lua

@@ -998,6 +998,14 @@ function M.add(text)
   log.info('Task added: ' .. description)
 end
 
+---@class pending.SyncBackend
+---@field name string
+---@field auth fun(): nil
+---@field push? fun(): nil
+---@field pull? fun(): nil
+---@field sync? fun(): nil
+---@field health? fun(): nil
+
 ---@type string[]?
 local _sync_backends = nil