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/background_tasks.py

@@ -31,6 +31,7 @@ class BackgroundTaskResponse(BaseModel):
     triggered_by: str | None
     error_message: str | None
     details: dict | None
+    celery_task_id: str | None = None  # Celery task ID for Flower linking
 
 
 class BackgroundTasksStatsResponse(BaseModel):
@@ -77,6 +78,7 @@ def _convert_import_to_response(job) -> BackgroundTaskResponse:
             "errors": job.error_count,
             "total_processed": job.total_processed,
         },
+        celery_task_id=getattr(job, "celery_task_id", None),
     )
 
 
@@ -105,6 +107,7 @@ def _convert_test_run_to_response(run) -> BackgroundTaskResponse:
             "pass_rate": run.pass_rate,
             "git_branch": run.git_branch,
         },
+        celery_task_id=getattr(run, "celery_task_id", None),
     )
 
 
@@ -141,6 +144,7 @@ def _convert_scan_to_response(scan) -> BackgroundTaskResponse:
             "git_commit_hash": scan.git_commit_hash,
             "progress_message": scan.progress_message,
         },
+        celery_task_id=getattr(scan, "celery_task_id", None),
     )