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/core/config.py

@@ -45,9 +45,9 @@ class Settings(BaseSettings):
     """
 
     # =============================================================================
-    # DATABASE
+    # DATABASE (PostgreSQL only)
     # =============================================================================
-    database_url: str = "sqlite:///./wizamart.db"
+    database_url: str = "postgresql://wizamart_user:secure_password@localhost:5432/wizamart_db"
 
     # =============================================================================
     # ADMIN INITIALIZATION (for init_production.py)
@@ -174,6 +174,19 @@ class Settings(BaseSettings):
     seed_products_per_vendor: int = 20  # Products per vendor
     seed_orders_per_vendor: int = 10  # Orders per vendor
 
+    # =============================================================================
+    # CELERY / REDIS TASK QUEUE
+    # =============================================================================
+    # Redis URL for Celery broker and result backend
+    redis_url: str = "redis://localhost:6379/0"
+
+    # Feature flag: enable Celery for background tasks (False = use FastAPI BackgroundTasks)
+    use_celery: bool = False
+
+    # Flower monitoring dashboard
+    flower_url: str = "http://localhost:5555"
+    flower_password: str = "changeme"  # CHANGE IN PRODUCTION!
+
     model_config = {"env_file": ".env"}
 
 
@@ -238,6 +251,27 @@ def is_staging_environment() -> bool:
 # =============================================================================
 
 
+def validate_database_url() -> None:
+    """
+    Validate that database URL is PostgreSQL.
+
+    Raises:
+        ValueError: If database URL is not PostgreSQL
+    """
+    if settings.database_url.startswith("sqlite"):
+        raise ValueError(
+            "SQLite is not supported. Please use PostgreSQL.\n"
+            "Set DATABASE_URL environment variable to a PostgreSQL connection string.\n"
+            "Example: postgresql://user:password@localhost:5432/dbname\n"
+            "For local development, run: docker-compose up -d db"
+        )
+    if not settings.database_url.startswith("postgresql"):
+        raise ValueError(
+            f"Unsupported database: {settings.database_url.split(':')[0]}\n"
+            "Only PostgreSQL is supported."
+        )
+
+
 def validate_production_settings() -> list[str]:
     """
     Validate settings for production environment.
@@ -304,6 +338,7 @@ __all__ = [
     "is_development_environment",
     "is_staging_environment",
     # Validation
+    "validate_database_url",
     "validate_production_settings",
     "print_environment_info",
 ]