refactor(sync): extract backend interface, adapt gcal module

Problem: :Pending sync hardcodes Google Calendar — M.sync() does
pcall(require, 'pending.sync.gcal') and calls gcal.sync() directly.
The config has a flat gcal field. This prevents adding new sync backends
without modifying init.lua.

Solution: Define a backend interface contract (name, auth, sync, health
fields), refactor :Pending sync to dispatch via require('pending.sync.'
.. backend_name), add sync table to config with legacy gcal migration,
rename gcal.authorize to gcal.auth, add gcal.health for checkhealth,
and add tab completion for backend names and actions.

lua/pending/config.lua

@@ -2,6 +2,9 @@
 ---@field calendar? string
 ---@field credentials_path? string
 
+---@class pending.SyncConfig
+---@field gcal? pending.GcalConfig
+
 ---@class pending.Keymaps
 ---@field close? string|false
 ---@field toggle? string|false
@@ -32,6 +35,7 @@
 ---@field drawer_height? integer
 ---@field debug? boolean
 ---@field keymaps pending.Keymaps
+---@field sync? pending.SyncConfig
 ---@field gcal? pending.GcalConfig
 
 ---@class pending.config
@@ -65,6 +69,7 @@ local defaults = {
     next_task = ']t',
     prev_task = '[t',
   },
+  sync = {},
 }
 
 ---@type pending.Config?
@@ -77,6 +82,10 @@ function M.get()
   end
   local user = vim.g.pending or {}
   _resolved = vim.tbl_deep_extend('force', defaults, user)
+  if _resolved.gcal and not (_resolved.sync and _resolved.sync.gcal) then
+    _resolved.sync = _resolved.sync or {}
+    _resolved.sync.gcal = _resolved.gcal
+  end
   return _resolved
 end