docs(sync): update vimdoc for backend interface

Problem: Vimdoc documents :Pending sync as a bare command that pushes
to Google Calendar, with no mention of backends or the sync table config.

Solution: Update :Pending sync section to show {backend} [{action}]
syntax with examples, add SYNC BACKENDS section documenting the interface
contract, update config example to use sync.gcal, document legacy gcal
migration, and update health check description.

doc/pending.txt

@@ -47,7 +47,7 @@ REQUIREMENTS                                           *pending-requirements*
 
 - Neovim 0.10+
 - No external dependencies for local use
-- `curl` and `openssl` are required for Google Calendar sync
+- `curl` and `openssl` are required for the `gcal` sync backend
 
 ==============================================================================
 INSTALL                                                      *pending-install*
@@ -250,10 +250,23 @@ COMMANDS                                                    *pending-commands*
     Open the list with |:copen| to navigate to each task's category.
 
                                                                *:Pending-sync*
-:Pending sync
-    Push pending tasks that have a due date to Google Calendar as all-day
-    events. Requires |pending-gcal| to be configured. See |pending-gcal| for
-    full details on what gets created, updated, and deleted.
+:Pending sync {backend} [{action}]
+    Run a sync action against a named backend. {backend} is required — bare
+    `:Pending sync` prints a usage message. {action} defaults to `sync`
+    when omitted. Each backend lives at `lua/pending/sync/<name>.lua`.
+
+    Examples: >vim
+        :Pending sync gcal          " runs gcal.sync()
+        :Pending sync gcal auth     " runs gcal.auth()
+        :Pending sync gcal sync     " explicit sync (same as bare)
+<
+
+    Tab completion after `:Pending sync ` lists discovered backends.
+    Tab completion after `:Pending sync gcal ` lists available actions.
+
+    Built-in backends: ~
+
+    `gcal`    Google Calendar one-way push. See |pending-gcal|.
 
                                                                *:Pending-undo*
 :Pending undo
@@ -440,9 +453,11 @@ loads: >lua
         next_task = ']t',
         prev_task = '[t',
       },
-      gcal = {
-        calendar = 'Tasks',
-        credentials_path = '/path/to/client_secret.json',
+      sync = {
+        gcal = {
+          calendar = 'Tasks',
+          credentials_path = '/path/to/client_secret.json',
+        },
       },
     }
 <
@@ -508,10 +523,16 @@ Fields: ~
                              vim.g.pending = { debug = true }
 <
 
+    {sync}               (table, default: {})          *pending.SyncConfig*
+                         Sync backend configuration. Each key is a backend
+                         name and the value is the backend-specific config
+                         table. Currently only `gcal` is built-in.
+
     {gcal}               (table, default: nil)
-                         Google Calendar sync configuration. See
-                         |pending.GcalConfig|. Omit this field entirely to
-                         disable Google Calendar sync.
+                         Legacy shorthand for `sync.gcal`. If `gcal` is set
+                         but `sync.gcal` is not, the value is migrated
+                         automatically. New configs should use `sync.gcal`
+                         instead. See |pending.GcalConfig|.
 
 ==============================================================================
 LUA API                                                          *pending-api*
@@ -632,13 +653,18 @@ not pulled back into pending.nvim.
 
 Configuration: >lua
     vim.g.pending = {
-      gcal = {
-        calendar = 'Tasks',
-        credentials_path = '/path/to/client_secret.json',
+      sync = {
+        gcal = {
+          calendar = 'Tasks',
+          credentials_path = '/path/to/client_secret.json',
+        },
       },
     }
 <
 
+The legacy `gcal` top-level key is still accepted and migrated automatically.
+New configurations should use `sync.gcal`.
+
                                                           *pending.GcalConfig*
 Fields: ~
     {calendar}           (string, default: 'Pendings')
@@ -654,7 +680,7 @@ Fields: ~
                          that Google provides or as a bare credentials object.
 
 OAuth flow: ~
-On the first `:Pending sync` call the plugin detects that no refresh token
+On the first `:Pending sync gcal` call the plugin detects that no refresh token
 exists and opens the Google authorization URL in the browser using
 |vim.ui.open()|. A temporary local HTTP server listens on port 18392 for the
 OAuth redirect. The PKCE (Proof Key for Code Exchange) flow is used —
@@ -664,7 +690,7 @@ authorization code is exchanged for tokens and the refresh token is stored at
 use the stored refresh token and refresh the access token automatically when
 it is about to expire.
 
-`:Pending sync` behavior: ~
+`:Pending sync gcal` behavior: ~
 For each task in the store:
 - A pending task with a due date and no existing event: a new all-day event is
   created and the event ID is stored in the task's `_extra` table.
@@ -677,6 +703,30 @@ For each task in the store:
 A summary notification is shown after sync: `created: N, updated: N,
 deleted: N`.
 
+==============================================================================
+SYNC BACKENDS                                           *pending-sync-backend*
+
+Sync backends are Lua modules under `lua/pending/sync/<name>.lua`. Each
+module returns a table conforming to the backend interface: >lua
+
+    ---@class pending.SyncBackend
+    ---@field name string
+    ---@field auth fun(): nil
+    ---@field sync fun(): nil
+    ---@field health? fun(): nil
+<
+
+Required fields: ~
+    {name}    Backend identifier (matches the filename).
+    {sync}    Main sync action. Called by `:Pending sync <name>`.
+    {auth}    Authorization flow. Called by `:Pending sync <name> auth`.
+
+Optional fields: ~
+    {health}  Called by `:checkhealth pending` to report backend-specific
+              diagnostics (e.g. checking for external tools).
+
+Backend-specific configuration goes under `sync.<name>` in |pending-config|.
+
 ==============================================================================
 HIGHLIGHT GROUPS                                          *pending-highlights*
 
@@ -728,8 +778,8 @@ Checks performed: ~
 - Whether the data directory exists (warning if not yet created)
 - Whether the data file exists and can be parsed; reports total task count
 - Validates recurrence specs on stored tasks
-- Whether `curl` is available (required for Google Calendar sync)
-- Whether `openssl` is available (required for OAuth PKCE)
+- Discovers sync backends under `lua/pending/sync/` and runs each backend's
+  `health()` function if it exists (e.g. gcal checks for `curl` and `openssl`)
 
 ==============================================================================
 DATA FORMAT                                                    *pending-data*