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/config.lua

@@ -2,6 +2,9 @@
 ---@field calendar? string
 ---@field credentials_path? string
 
+---@class pending.SyncConfig
+---@field gcal? pending.GcalConfig
+
 ---@class pending.Keymaps
 ---@field close? string|false
 ---@field toggle? string|false
@@ -32,6 +35,7 @@
 ---@field drawer_height? integer
 ---@field debug? boolean
 ---@field keymaps pending.Keymaps
+---@field sync? pending.SyncConfig
 ---@field gcal? pending.GcalConfig
 
 ---@class pending.config
@@ -65,6 +69,7 @@ local defaults = {
     next_task = ']t',
     prev_task = '[t',
   },
+  sync = {},
 }
 
 ---@type pending.Config?
@@ -77,6 +82,10 @@ function M.get()
   end
   local user = vim.g.pending or {}
   _resolved = vim.tbl_deep_extend('force', defaults, user)
+  if _resolved.gcal and not (_resolved.sync and _resolved.sync.gcal) then
+    _resolved.sync = _resolved.sync or {}
+    _resolved.sync.gcal = _resolved.gcal
+  end
   return _resolved
 end
 

lua/pending/health.lua

@@ -47,16 +47,18 @@ function M.check()
     vim.health.info('No data file yet (will be created on first save)')
   end
 
-  if vim.fn.executable('curl') == 1 then
-    vim.health.ok('curl found (required for Google Calendar sync)')
+  local sync_paths = vim.fn.globpath(vim.o.runtimepath, 'lua/pending/sync/*.lua', false, true)
+  if #sync_paths == 0 then
+    vim.health.info('No sync backends found')
   else
-    vim.health.warn('curl not found (needed for Google Calendar sync)')
-  end
-
-  if vim.fn.executable('openssl') == 1 then
-    vim.health.ok('openssl found (required for OAuth PKCE)')
-  else
-    vim.health.warn('openssl not found (needed for Google Calendar OAuth)')
+    for _, path in ipairs(sync_paths) do
+      local name = vim.fn.fnamemodify(path, ':t:r')
+      local bok, backend = pcall(require, 'pending.sync.' .. name)
+      if bok and type(backend.health) == 'function' then
+        vim.health.start('pending.nvim: sync/' .. name)
+        backend.health()
+      end
+    end
   end
 end
 

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)

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

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,
 })