feat(phase8.3): recover_stalled_renders beat task

Detect OrderLines stuck in render_status='processing' for >30min,
revoke their Celery task, mark failed, update render_job_doc, and
emit a single CHANNEL_ALERT batch notification to admins.

Runs every 5min via beat schedule alongside recover_stuck_cad_files.
Section B (tenant_id in JWT) confirmed already complete from Phase 4.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/tasks/celery_app.py

@@ -42,5 +42,9 @@ celery_app.conf.update(
             "task": "app.tasks.beat_tasks.batch_render_notifications",
             "schedule": 60.0,  # every 60 seconds
         },
+        "recover-stalled-renders-every-5m": {
+            "task": "app.tasks.beat_tasks.recover_stalled_renders",
+            "schedule": 300.0,  # every 5 minutes
+        },
     },
 )