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/test_run.py

@@ -56,6 +56,9 @@ class TestRun(Base):
         String(20), default="running", index=True
     )  # 'running', 'passed', 'failed', 'error'
 
+    # Celery task tracking (optional - for USE_CELERY=true)
+    celery_task_id = Column(String(255), nullable=True, index=True)
+
     # Relationship to test results
     results = relationship(
         "TestResult", back_populates="run", cascade="all, delete-orphan"