feat(sync): auto-discover backends, per-backend auth, S3 backend

Problem: sync backends were hardcoded in `SYNC_BACKENDS` list in
`init.lua`, auth routed directly through `oauth.google_client`, and
adding a non-OAuth backend required editing multiple files.

Solution: replace hardcoded list with `discover_backends()` that globs
`lua/pending/sync/*.lua` at runtime. Rewrite `M.auth()` to dispatch
to per-backend `auth()` methods with `vim.ui.select` fallback. Add
`lua/pending/sync/s3.lua` with push/pull/sync via AWS CLI, per-task
merge by `_s3_sync_id` (UUID), and `pending.S3Config` type.

lua/pending/sync/util.lua

@@ -5,6 +5,10 @@ local log = require('pending.log')
 ---@field stdout string
 ---@field stderr string
 
+---@class pending.CountPart
+---@field [1] integer
+---@field [2] string
+
 ---@class pending.sync.util
 local M = {}
 
@@ -61,7 +65,7 @@ function M.finish(s)
   end
 end
 
----@param parts [integer, string][]
+---@param parts pending.CountPart[]
 ---@return string
 function M.fmt_counts(parts)
   local items = {}