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/celery_config.py

@@ -0,0 +1,124 @@
+# app/core/celery_config.py
+"""
+Celery configuration for Wizamart background task processing.
+
+This module configures Celery with Redis as the broker and result backend.
+It includes:
+- Task routing to separate queues (default, long_running, scheduled)
+- Celery Beat schedule for periodic tasks
+- Task retry policies
+"""
+
+import os
+
+from celery import Celery
+from celery.schedules import crontab
+
+# Redis URL from environment or default
+REDIS_URL = os.getenv("REDIS_URL", "redis://localhost:6379/0")
+
+# Create Celery application
+celery_app = Celery(
+    "wizamart",
+    broker=REDIS_URL,
+    backend=REDIS_URL,
+    include=[
+        "app.tasks.celery_tasks.marketplace",
+        "app.tasks.celery_tasks.letzshop",
+        "app.tasks.celery_tasks.subscription",
+        "app.tasks.celery_tasks.export",
+        "app.tasks.celery_tasks.code_quality",
+        "app.tasks.celery_tasks.test_runner",
+    ],
+)
+
+# =============================================================================
+# CELERY CONFIGURATION
+# =============================================================================
+celery_app.conf.update(
+    # Serialization
+    task_serializer="json",
+    accept_content=["json"],
+    result_serializer="json",
+    # Timezone
+    timezone="Europe/Luxembourg",
+    enable_utc=True,
+    # Task behavior
+    task_track_started=True,
+    task_time_limit=30 * 60,  # 30 minutes hard limit
+    task_soft_time_limit=25 * 60,  # 25 minutes soft limit
+    # Worker settings
+    worker_prefetch_multiplier=1,  # Disable prefetching for long tasks
+    worker_concurrency=4,  # Number of concurrent workers
+    # Result backend
+    result_expires=86400,  # Results expire after 24 hours
+    # Retry policy
+    task_default_retry_delay=60,  # 1 minute between retries
+    task_max_retries=3,
+    # Task events (required for Flower monitoring)
+    worker_send_task_events=True,
+    task_send_sent_event=True,
+)
+
+# =============================================================================
+# TASK ROUTING - Route tasks to appropriate queues
+# =============================================================================
+celery_app.conf.task_routes = {
+    # Long-running import tasks
+    "app.tasks.celery_tasks.marketplace.*": {"queue": "long_running"},
+    "app.tasks.celery_tasks.letzshop.*": {"queue": "long_running"},
+    # Fast export tasks
+    "app.tasks.celery_tasks.export.*": {"queue": "default"},
+    # Scheduled subscription tasks
+    "app.tasks.celery_tasks.subscription.*": {"queue": "scheduled"},
+    # Code quality and test tasks (can be long)
+    "app.tasks.celery_tasks.code_quality.*": {"queue": "long_running"},
+    "app.tasks.celery_tasks.test_runner.*": {"queue": "long_running"},
+}
+
+# =============================================================================
+# CELERY BEAT SCHEDULE - Periodic tasks
+# =============================================================================
+celery_app.conf.beat_schedule = {
+    # Reset usage counters at start of each period
+    "reset-period-counters-daily": {
+        "task": "app.tasks.celery_tasks.subscription.reset_period_counters",
+        "schedule": crontab(hour=0, minute=5),  # 00:05 daily
+        "options": {"queue": "scheduled"},
+    },
+    # Check for expiring trials and send notifications
+    "check-trial-expirations-daily": {
+        "task": "app.tasks.celery_tasks.subscription.check_trial_expirations",
+        "schedule": crontab(hour=1, minute=0),  # 01:00 daily
+        "options": {"queue": "scheduled"},
+    },
+    # Sync subscription status with Stripe
+    "sync-stripe-status-hourly": {
+        "task": "app.tasks.celery_tasks.subscription.sync_stripe_status",
+        "schedule": crontab(minute=30),  # Every hour at :30
+        "options": {"queue": "scheduled"},
+    },
+    # Clean up stale/orphaned subscriptions
+    "cleanup-stale-subscriptions-weekly": {
+        "task": "app.tasks.celery_tasks.subscription.cleanup_stale_subscriptions",
+        "schedule": crontab(hour=3, minute=0, day_of_week=0),  # Sunday 03:00
+        "options": {"queue": "scheduled"},
+    },
+    # Capture daily capacity snapshot for analytics
+    "capture-capacity-snapshot-daily": {
+        "task": "app.tasks.celery_tasks.subscription.capture_capacity_snapshot",
+        "schedule": crontab(hour=0, minute=0),  # Midnight daily
+        "options": {"queue": "scheduled"},
+    },
+}
+
+# =============================================================================
+# QUEUE CONFIGURATION
+# =============================================================================
+celery_app.conf.task_queues = {
+    "default": {"exchange": "default", "routing_key": "default"},
+    "long_running": {"exchange": "long_running", "routing_key": "long_running"},
+    "scheduled": {"exchange": "scheduled", "routing_key": "scheduled"},
+}
+
+celery_app.conf.task_default_queue = "default"