refactor: tighten LuaCATS annotations and canonicalize metadata fields (#141)

* 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`.

* refactor: canonicalize internal metadata field names

Problem: `pending.Metadata` used shorthand field names (`cat`, `rec`,
`rec_mode`) matching user-facing token syntax, coupling internal
representation to config. `RecurSpec.from_completion` used a boolean
where a `pending.RecurMode` alias exists. `category_syntax` was
hardcoded to `'cat'` with no config option.

Solution: rename `Metadata` fields to `category`/`recur`/`recur_mode`,
add `category_syntax` config option (default `'cat'`), rename
`ParsedEntry` fields to match, replace `RecurSpec.from_completion`
with `mode: pending.RecurMode`, and restore `[string]` indexer on
`pending.ForgeConfig` alongside explicit fields.

lua/pending/recur.lua

@@ -2,7 +2,7 @@
 ---@field freq 'daily'|'weekly'|'monthly'|'yearly'
 ---@field interval integer
 ---@field byday? string[]
----@field from_completion boolean
+---@field mode pending.RecurMode
 ---@field _raw? string
 
 ---@class pending.recur
@@ -10,29 +10,29 @@ local M = {}
 
 ---@type table<string, pending.RecurSpec>
 local named = {
-  daily = { freq = 'daily', interval = 1, from_completion = false },
+  daily = { freq = 'daily', interval = 1, mode = 'scheduled' },
   weekdays = {
     freq = 'weekly',
     interval = 1,
     byday = { 'MO', 'TU', 'WE', 'TH', 'FR' },
-    from_completion = false,
+    mode = 'scheduled',
   },
-  weekly = { freq = 'weekly', interval = 1, from_completion = false },
-  biweekly = { freq = 'weekly', interval = 2, from_completion = false },
-  monthly = { freq = 'monthly', interval = 1, from_completion = false },
-  quarterly = { freq = 'monthly', interval = 3, from_completion = false },
-  yearly = { freq = 'yearly', interval = 1, from_completion = false },
-  annual = { freq = 'yearly', interval = 1, from_completion = false },
+  weekly = { freq = 'weekly', interval = 1, mode = 'scheduled' },
+  biweekly = { freq = 'weekly', interval = 2, mode = 'scheduled' },
+  monthly = { freq = 'monthly', interval = 1, mode = 'scheduled' },
+  quarterly = { freq = 'monthly', interval = 3, mode = 'scheduled' },
+  yearly = { freq = 'yearly', interval = 1, mode = 'scheduled' },
+  annual = { freq = 'yearly', interval = 1, mode = 'scheduled' },
 }
 
 ---@param spec string
 ---@return pending.RecurSpec?
 function M.parse(spec)
-  local from_completion = false
+  local mode = 'scheduled' ---@type pending.RecurMode
   local s = spec
 
   if s:sub(1, 1) == '!' then
-    from_completion = true
+    mode = 'completion'
     s = s:sub(2)
   end
 
@@ -44,7 +44,7 @@ function M.parse(spec)
       freq = base.freq,
       interval = base.interval,
       byday = base.byday,
-      from_completion = from_completion,
+      mode = mode,
     }
   end
 
@@ -58,7 +58,7 @@ function M.parse(spec)
     return {
       freq = freq_map[unit],
       interval = num,
-      from_completion = from_completion,
+      mode = mode,
     }
   end
 
@@ -66,7 +66,7 @@ function M.parse(spec)
     return {
       freq = 'daily',
       interval = 1,
-      from_completion = from_completion,
+      mode = mode,
       _raw = s,
     }
   end
@@ -134,7 +134,7 @@ end
 
 ---@param base_date string
 ---@param spec string
----@param mode 'scheduled'|'completion'
+---@param mode pending.RecurMode
 ---@return string
 function M.next_due(base_date, spec, mode)
   local parsed = M.parse(spec)