feat: auth backend (#111)

* refactor(types): extract inline anonymous types into named classes

Problem: several functions used inline `{...}` table types in their
`@param` and `@return` annotations, making them hard to read and
impossible to reference from other modules.

Solution: extract each into a named `---@class`: `pending.Metadata`,
`pending.TaskFields`, `pending.CompletionItem`, `pending.SystemResult`,
and `pending.OAuthClientOpts`.

* refactor(sync): extract shared utilities into `sync/util.lua`

Problem: sync epilogue code (`s:save()`, `_recompute_counts()`,
`buffer.render()`) and `fmt_counts` were duplicated across `gcal.lua`
and `gtasks.lua`. The concurrency guard lived in `oauth.lua`, coupling
non-OAuth backends to the OAuth module.

Solution: create `sync/util.lua` with `async`, `system`, `with_guard`,
`finish`, and `fmt_counts`. Delegate from `oauth.lua` and replace
duplicated code in both backends. Add per-backend `auth()` and
`auth_complete()` methods to `gcal.lua` and `gtasks.lua`.

* 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 = {}