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/config.lua

@@ -27,10 +27,17 @@
 ---@field client_id? string
 ---@field client_secret? string
 
+---@class pending.S3Config
+---@field bucket string
+---@field key? string
+---@field profile? string
+---@field region? string
+
 ---@class pending.SyncConfig
 ---@field remote_delete? boolean
 ---@field gcal? pending.GcalConfig
 ---@field gtasks? pending.GtasksConfig
+---@field s3? pending.S3Config
 
 ---@class pending.Keymaps
 ---@field close? string|false