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/store.lua

@@ -1,19 +1,31 @@
 local config = require('pending.config')
 
+---@alias pending.TaskStatus 'pending'|'done'|'deleted'|'wip'|'blocked'
+---@alias pending.RecurMode 'scheduled'|'completion'
+
+---@class pending.TaskExtra
+---@field _forge_ref? pending.ForgeRef
+---@field _forge_cache? pending.ForgeCache
+---@field _gtasks_task_id? string
+---@field _gtasks_list_id? string
+---@field _gcal_event_id? string
+---@field _gcal_calendar_id? string
+---@field [string] any
+
 ---@class pending.Task
 ---@field id integer
 ---@field description string
----@field status 'pending'|'done'|'deleted'|'wip'|'blocked'
+---@field status pending.TaskStatus
 ---@field category? string
 ---@field priority integer
 ---@field due? string
 ---@field recur? string
----@field recur_mode? 'scheduled'|'completion'
+---@field recur_mode? pending.RecurMode
 ---@field entry string
 ---@field modified string
 ---@field end? string
 ---@field order integer
----@field _extra? table<string, any>
+---@field _extra? pending.TaskExtra
 
 ---@class pending.Data
 ---@field version integer
@@ -24,14 +36,14 @@ local config = require('pending.config')
 
 ---@class pending.TaskFields
 ---@field description string
----@field status? string
+---@field status? pending.TaskStatus
 ---@field category? string
 ---@field priority? integer
 ---@field due? string
 ---@field recur? string
----@field recur_mode? string
+---@field recur_mode? pending.RecurMode
 ---@field order? integer
----@field _extra? table
+---@field _extra? pending.TaskExtra
 
 ---@class pending.Store
 ---@field path string