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>

models/database/letzshop.py

@@ -213,6 +213,9 @@ class LetzshopHistoricalImportJob(Base, TimestampMixin):
     # Error handling
     error_message = Column(Text, nullable=True)
 
+    # Celery task tracking (optional - for USE_CELERY=true)
+    celery_task_id = Column(String(255), nullable=True, index=True)
+
     # Timing
     started_at = Column(DateTime(timezone=True), nullable=True)
     completed_at = Column(DateTime(timezone=True), nullable=True)