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.

lua/pending/init.lua

@@ -414,14 +414,25 @@ function M.add(text)
   vim.notify('Pending added: ' .. description)
 end
 
+---@param backend_name string
+---@param action? string
 ---@return nil
-function M.sync()
-  local ok, gcal = pcall(require, 'pending.sync.gcal')
-  if not ok then
-    vim.notify('Google Calendar sync module not available.', vim.log.levels.ERROR)
+function M.sync(backend_name, action)
+  if not backend_name or backend_name == '' then
+    vim.notify('Usage: :Pending sync <backend> [action]', vim.log.levels.ERROR)
     return
   end
-  gcal.sync()
+  action = (action and action ~= '') and action or 'sync'
+  local ok, backend = pcall(require, 'pending.sync.' .. backend_name)
+  if not ok then
+    vim.notify('Unknown sync backend: ' .. backend_name, vim.log.levels.ERROR)
+    return
+  end
+  if type(backend[action]) ~= 'function' then
+    vim.notify(backend_name .. " backend has no '" .. action .. "' action", vim.log.levels.ERROR)
+    return
+  end
+  backend[action]()
 end
 
 ---@param days? integer
@@ -700,7 +711,8 @@ function M.command(args)
     local id_str, edit_rest = rest:match('^(%S+)%s*(.*)')
     M.edit(id_str, edit_rest)
   elseif cmd == 'sync' then
-    M.sync()
+    local backend, action = rest:match('^(%S+)%s*(.*)')
+    M.sync(backend, action)
   elseif cmd == 'archive' then
     local d = rest ~= '' and tonumber(rest) or nil
     M.archive(d)