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/admin/letzshop.py

@@ -823,13 +823,20 @@ def start_historical_import(
 
     logger.info(f"Created historical import job {job.id} for vendor {vendor_id}")
 
-    # Queue background task
-    background_tasks.add_task(
-        process_historical_import,
-        job.id,
-        vendor_id,
+    # Dispatch via task dispatcher (supports Celery or BackgroundTasks)
+    from app.tasks.dispatcher import task_dispatcher
+
+    celery_task_id = task_dispatcher.dispatch_historical_import(
+        background_tasks=background_tasks,
+        job_id=job.id,
+        vendor_id=vendor_id,
     )
 
+    # Store Celery task ID if using Celery
+    if celery_task_id:
+        job.celery_task_id = celery_task_id
+        db.commit()
+
     return LetzshopHistoricalImportStartResponse(
         job_id=job.id,
         status="pending",