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>

.env.example

@@ -11,13 +11,13 @@ DESCRIPTION=Multi-tenants multi-themes ecommerce application
 VERSION=2.2.0
 
 # =============================================================================
-# DATABASE CONFIGURATION
+# DATABASE CONFIGURATION (PostgreSQL required)
 # =============================================================================
-# For development (SQLite)
-DATABASE_URL=sqlite:///./wizamart.db
+# Default works with: docker-compose up -d db
+DATABASE_URL=postgresql://wizamart_user:secure_password@localhost:5432/wizamart_db
 
-# For production (PostgreSQL)
-# DATABASE_URL=postgresql://username:password@localhost:5432/wizamart_db
+# For production, use your PostgreSQL connection string:
+# DATABASE_URL=postgresql://username:password@production-host:5432/wizamart_db
 
 # =============================================================================
 # ADMIN INITIALIZATION
@@ -145,3 +145,21 @@ SEED_DEMO_VENDORS=3
 SEED_CUSTOMERS_PER_VENDOR=15
 SEED_PRODUCTS_PER_VENDOR=20
 SEED_ORDERS_PER_VENDOR=10
+
+# =============================================================================
+# CELERY / REDIS TASK QUEUE
+# =============================================================================
+# Redis connection URL (used for Celery broker and backend)
+# Default works with: docker-compose up -d redis
+REDIS_URL=redis://localhost:6379/0
+
+# Enable Celery for background tasks (set to false to use FastAPI BackgroundTasks)
+# Recommended: false for development, true for production
+USE_CELERY=false
+
+# Flower monitoring dashboard URL (for admin panel links)
+FLOWER_URL=http://localhost:5555
+
+# Flower basic authentication password
+# ⚠️  CHANGE THIS IN PRODUCTION!
+FLOWER_PASSWORD=changeme