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/sync/gcal.lua

@@ -3,6 +3,8 @@ local store = require('pending.store')
 
 local M = {}
 
+M.name = 'gcal'
+
 local BASE_URL = 'https://www.googleapis.com/calendar/v3'
 local TOKEN_URL = 'https://oauth2.googleapis.com/token'
 local AUTH_URL = 'https://accounts.google.com/o/oauth2/v2/auth'
@@ -22,7 +24,7 @@ local SCOPE = 'https://www.googleapis.com/auth/calendar'
 ---@return table<string, any>
 local function gcal_config()
   local cfg = config.get()
-  return cfg.gcal or {}
+  return (cfg.sync and cfg.sync.gcal) or cfg.gcal or {}
 end
 
 ---@return string
@@ -199,7 +201,7 @@ local function get_access_token()
   end
   local tokens = load_tokens()
   if not tokens or not tokens.refresh_token then
-    M.authorize()
+    M.auth()
     tokens = load_tokens()
     if not tokens then
       return nil
@@ -218,7 +220,7 @@ local function get_access_token()
   return tokens.access_token
 end
 
-function M.authorize()
+function M.auth()
   local creds = load_credentials()
   if not creds then
     vim.notify(
@@ -514,4 +516,18 @@ function M.sync()
   )
 end
 
+---@return nil
+function M.health()
+  if vim.fn.executable('curl') == 1 then
+    vim.health.ok('curl found (required for gcal sync)')
+  else
+    vim.health.warn('curl not found (needed for gcal sync)')
+  end
+  if vim.fn.executable('openssl') == 1 then
+    vim.health.ok('openssl found (required for gcal OAuth PKCE)')
+  else
+    vim.health.warn('openssl not found (needed for gcal OAuth)')
+  end
+end
+
 return M