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/api/v1/admin/vendors.py

@@ -382,24 +382,46 @@ def export_vendor_products_letzshop_to_folder(
     automated product sync.
 
     **Behavior:**
+    - When Celery is enabled: Queues export as background task, returns immediately
+    - When Celery is disabled: Runs synchronously and returns file paths
     - Creates CSV files for each language (fr, de, en)
     - Places files in: exports/letzshop/{vendor_code}/
     - Filename format: {vendor_code}_products_{language}.csv
 
     Returns:
-        JSON with export status and file paths
+        JSON with export status and file paths (or task_id if async)
     """
     import os
     from datetime import UTC, datetime
     from pathlib import Path as FilePath
 
+    from app.core.config import settings
     from app.services.letzshop_export_service import letzshop_export_service
 
     vendor = vendor_service.get_vendor_by_identifier(db, vendor_identifier)
-
     include_inactive = request.include_inactive if request else False
 
-    # Track export start time
+    # If Celery is enabled, dispatch as async task
+    if settings.use_celery:
+        from app.tasks.dispatcher import task_dispatcher
+
+        celery_task_id = task_dispatcher.dispatch_product_export(
+            vendor_id=vendor.id,
+            triggered_by=f"admin:{current_admin.id}",
+            include_inactive=include_inactive,
+        )
+
+        return {
+            "success": True,
+            "message": f"Export task queued for vendor {vendor.vendor_code}. Check Flower for status.",
+            "vendor_code": vendor.vendor_code,
+            "export_directory": f"exports/letzshop/{vendor.vendor_code.lower()}",
+            "files": [],
+            "celery_task_id": celery_task_id,
+            "is_async": True,
+        }
+
+    # Synchronous export (when Celery is disabled)
     started_at = datetime.now(UTC)
 
     # Create export directory
@@ -445,7 +467,6 @@ def export_vendor_products_letzshop_to_folder(
             })
 
     # Log the export operation via service
-    # For exports, track files (languages) as the unit of work
     completed_at = datetime.now(UTC)
     letzshop_export_service.log_export(
         db=db,
@@ -467,4 +488,5 @@ def export_vendor_products_letzshop_to_folder(
         "vendor_code": vendor.vendor_code,
         "export_directory": str(export_dir),
         "files": exported_files,
+        "is_async": False,
     }