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/tasks/celery_tasks/__init__.py

@@ -0,0 +1,16 @@
+# app/tasks/celery_tasks/__init__.py
+"""
+Celery task modules for Wizamart.
+
+This package contains Celery task wrappers for background processing:
+- marketplace: Product import tasks
+- letzshop: Historical import tasks
+- subscription: Scheduled subscription management
+- export: Product export tasks
+- code_quality: Code quality scan tasks
+- test_runner: Test execution tasks
+"""
+
+from app.tasks.celery_tasks.base import DatabaseTask
+
+__all__ = ["DatabaseTask"]