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.

plugin/pending.lua

@@ -171,6 +171,34 @@ end, {
     if cmd_line:match('^Pending%s+edit') then
       return complete_edit(arg_lead, cmd_line)
     end
+    if cmd_line:match('^Pending%s+sync') then
+      local after_sync = cmd_line:match('^Pending%s+sync%s+(.*)')
+      if not after_sync then
+        return {}
+      end
+      local parts = {}
+      for part in after_sync:gmatch('%S+') do
+        table.insert(parts, part)
+      end
+      local trailing_space = after_sync:match('%s$')
+      if #parts == 0 or (#parts == 1 and not trailing_space) then
+        local backends = {}
+        local pattern = vim.fn.globpath(vim.o.runtimepath, 'lua/pending/sync/*.lua', false, true)
+        for _, path in ipairs(pattern) do
+          local name = vim.fn.fnamemodify(path, ':t:r')
+          table.insert(backends, name)
+        end
+        table.sort(backends)
+        return filter_candidates(arg_lead, backends)
+      end
+      if #parts == 1 and trailing_space then
+        return filter_candidates(arg_lead, { 'auth', 'sync' })
+      end
+      if #parts >= 2 and not trailing_space then
+        return filter_candidates(arg_lead, { 'auth', 'sync' })
+      end
+      return {}
+    end
     return {}
   end,
 })