fix(celery): bind @shared_task to our app so async dispatch reaches redis

When the FastAPI process called send_notification_email.delay() it
crashed with kombu OperationalError "[Errno 111] Connection refused"
because the published task was going to amqp://localhost// instead
of our configured redis broker.

Root cause: tasks decorated with @shared_task bind to whatever
Celery considers the "current default app" at decoration time. Our
celery_app (with redis broker) was never imported during FastAPI
startup, so the loyalty / billing / etc. task modules registered
their @shared_task functions against Celery's built-in default —
which has broker_url=amqp://localhost// and no RabbitMQ deployed.

Two-part fix:

  1. app/core/celery_config.py — call celery_app.set_default() so
     any @shared_task evaluated after this module loads binds to our
     app rather than the built-in default.

  2. main.py — import app.core.celery_config near the top, BEFORE
     api_router (which transitively imports the task modules). The
     side-effect of the import (creating celery_app + set_default())
     must run before any @shared_task decorator fires. An # isort:
     split below the import stops the import-sorter from alphabetising
     it back behind app.api.main and re-introducing the bug.

User-visible effect: loyalty welcome emails (and every other
@shared_task-based notification) now actually queue + send.
Surfaced this while debugging why enrollment on prod produced a
loyalty_card row but no email_logs row.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/core/celery_config.py

@@ -72,6 +72,12 @@ celery_app = Celery(
     include=get_all_task_modules(),
 )
 
+# Mark this as the default app so @shared_task decorators in module task
+# files bind to it. Otherwise they fall back to Celery's built-in default,
+# which uses amqp://localhost// and silently fails with "Connection refused"
+# at .delay() time when no RabbitMQ is deployed.
+celery_app.set_default()
+
 # =============================================================================
 # CELERY CONFIGURATION
 # =============================================================================

main.py

@@ -32,6 +32,16 @@ from sqlalchemy import text
 from sqlalchemy.orm import Session
 from uvicorn.middleware.proxy_headers import ProxyHeadersMiddleware
 
+# Bootstrap Celery default app BEFORE importing api_router, which
+# transitively imports loyalty/billing/etc. task modules. Their @shared_task
+# decorators bind at import time to whatever Celery considers the current
+# default app — without our app.set_default() running first, they fall back
+# to Celery's built-in default (amqp://localhost//) and .delay() raises
+# ConnectionRefusedError at runtime. The isort: split below stops ruff from
+# alphabetically reordering this back behind api_router.
+from app.core import celery_config  # noqa: F401  # side-effect: set_default()
+
+# isort: split
 from app.api.main import api_router
 from app.core.config import settings