feat: Add daily Threads content generation for manual publishing

- Add generate_threads_manual_posts() task that creates 2 posts daily
- Posts are saved with status 'draft' for manual copy/paste to Threads
- Uses tip_tech template with rotating categories (productividad, ia, etc.)
- Scheduled to run daily at 7:00 AM (Tijuana timezone)
- Posts appear in dashboard for easy access

This is a workaround while waiting for Threads API approval.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

worker/celery_app.py

@@ -79,6 +79,13 @@ celery_app.conf.beat_schedule = {
         "schedule": crontab(day_of_week=0, hour=5, minute=0),  # Domingos 5 AM
     },
 
+    # Generar posts para Threads (publicación manual) - 7:00 AM
+    "generate-threads-manual": {
+        "task": "worker.tasks.generate_content.generate_threads_manual_posts",
+        "schedule": crontab(hour=7, minute=0),
+        "args": [2],  # 2 posts diarios
+    },
+
     # Reporte matutino por Telegram (8:00 AM Tijuana)
     "telegram-morning-report": {
         "task": "worker.tasks.daily_reports.morning_report",