feat: queue processor — scheduled task + page-load fallback (#11)

Three-pronged queue processing:

1. Joomla Scheduled Task (preferred): New plg_task_mokojoomcross plugin
   registers "MokoJoomCross - Process Queue" task type. Admin creates
   a scheduled task in System → Scheduled Tasks with desired interval.

2. Page-load fallback: System plugin onAfterRender with configurable
   throttle interval. Runs on backend, frontend, or both. Small batch
   size (5) to avoid slowing page loads.

3. Both can run simultaneously — QueueProcessor uses DB-based lock
   to prevent concurrent execution (120s safety timeout).

Shared QueueProcessor helper handles:
- Queued post dispatch to service plugins
- Failed post retry with configurable max retries + delay
- Scheduled post firing (when scheduled_at <= now)
- Log cleanup based on retention period

Dashboard shows warning banner when page-load processing is active,
recommending switch to Joomla Scheduled Tasks for production.

Config options: queue_processing (scheduler/pageload/both),
pageload_client (admin/site/both), pageload_interval (seconds).

Authored-by: Moko Consulting

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/script.php

@@ -63,6 +63,7 @@ class Pkg_MokoJoomCrossInstallerScript
                 ['system', 'mokojoomcross'],
                 ['content', 'mokojoomcross'],
                 ['webservices', 'mokojoomcross'],
+                ['task', 'mokojoomcross'],
             ];
 
             foreach ($corePlugins as [$folder, $element]) {