fix: resolve OAuth re-auth deadlock and sync concurrency races (#88)

* fix(gtasks): prevent concurrent push/pull from racing on the store

Problem: `push` and `pull` both run via `oauth.async`, so issuing them
back-to-back starts two coroutines that interleave at every curl yield.
Both snapshot `build_id_index` before either has mutated the store,
which can cause push to create a remote task that pull would have
recognized as already linked, producing duplicates on Google.

Solution: guard `with_token` with a module-level `_in_flight` flag set
before `oauth.async` is called so no second operation can start during
a token-refresh yield. A `pcall` around the callback guarantees the
flag is always cleared, even on an unexpected error.

* refactor(sync): centralize `with_token` in oauth.lua with shared lock

Problem: `with_token` was duplicated in `gcal.lua` and `gtasks.lua`,
with the concurrency lock added only to the gtasks copy. Any new
backend would silently inherit the same race, and gcal back-to-back
push could still create duplicate remote calendar events.

Solution: lift `with_token` into `oauth.lua` as
`M.with_token(client, name, callback)` behind a module-level
`_sync_in_flight` guard. All backends share one implementation; the
lock covers gcal, gtasks, and any future backend automatically.

* ci: format

lua/pending/sync/oauth.lua

@@ -26,6 +26,7 @@ local OAuthClient = {}
 OAuthClient.__index = OAuthClient
 
 local _active_close = nil
+local _sync_in_flight = false
 
 ---@class pending.oauth
 local M = {}
@@ -51,6 +52,30 @@ function M.async(fn)
   coroutine.resume(coroutine.create(fn))
 end
 
+---@param client pending.OAuthClient
+---@param name string
+---@param callback fun(access_token: string): nil
+function M.with_token(client, name, callback)
+  if _sync_in_flight then
+    require('pending.log').warn(name .. ': sync operation in progress — please wait')
+    return
+  end
+  _sync_in_flight = true
+  M.async(function()
+    local token = client:get_access_token()
+    if not token then
+      _sync_in_flight = false
+      require('pending.log').warn(name .. ': not authenticated — run :Pending auth')
+      return
+    end
+    local ok, err = pcall(callback, token)
+    _sync_in_flight = false
+    if not ok then
+      require('pending.log').error(name .. ': ' .. tostring(err))
+    end
+  end)
+end
+
 ---@param str string
 ---@return string
 function M.url_encode(str)