feat: add Celery/Redis task queue with feature flag support

Migrate background tasks from FastAPI BackgroundTasks to Celery with Redis
for persistent task queuing, retries, and scheduled jobs.

Key changes:
- Add Celery configuration with Redis broker/backend
- Create task dispatcher with USE_CELERY feature flag for gradual rollout
- Add Celery task wrappers for all background operations:
  - Marketplace imports
  - Letzshop historical imports
  - Product exports
  - Code quality scans
  - Test runs
  - Subscription scheduled tasks (via Celery Beat)
- Add celery_task_id column to job tables for Flower integration
- Add Flower dashboard link to admin background tasks page
- Update docker-compose.yml with worker, beat, and flower services
- Add Makefile targets: celery-worker, celery-beat, celery-dev, flower

When USE_CELERY=false (default), system falls back to FastAPI BackgroundTasks
for development without Redis dependency.

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

app/api/v1/vendor/onboarding.py

@@ -227,11 +227,27 @@ def trigger_order_sync(
 
     # Queue background task to process the import
     if result.get("success") and result.get("job_id"):
-        background_tasks.add_task(
-            process_historical_import,
-            result["job_id"],
-            current_user.token_vendor_id,
+        from app.tasks.dispatcher import task_dispatcher
+
+        celery_task_id = task_dispatcher.dispatch_historical_import(
+            background_tasks=background_tasks,
+            job_id=result["job_id"],
+            vendor_id=current_user.token_vendor_id,
         )
+
+        # Store Celery task ID if using Celery
+        if celery_task_id:
+            from models.database.letzshop import LetzshopHistoricalImportJob
+
+            job = (
+                db.query(LetzshopHistoricalImportJob)
+                .filter(LetzshopHistoricalImportJob.id == result["job_id"])
+                .first()
+            )
+            if job:
+                job.celery_task_id = celery_task_id
+                db.commit()
+
         logger.info(f"Queued historical import task for job {result['job_id']}")
 
     return result