(fix) Celery DB pool cleanup

[Uxio0]

Summary

This change fixes the production regression introduced by the Celery DB pool cleanup work and hardens startup behavior around gevent.

The root cause was an import side effect chain in the config package:

gunicorn.conf.py -> config.gunicorn -> config.init -> config.celery_app

That caused the web pod to import Celery code before Gunicorn’s gevent worker had applied monkey patching. The new task_postrun cleanup in config.celery_app imported django.db, and under gevent that led to Django DB connections being created/closed under
mismatched thread identities in web requests.

Changes

• Removed the eager Celery import from config/init.py
• Kept the Celery task_postrun DB connection cleanup, but made the django.db import lazy inside the signal handler
• Added regression tests covering:
  • importing config.celery_app does not import django.db
  • the Celery post-task cleanup still closes non-atomic DB connections

Why

• Web startup should not import the Celery app as a package side effect
• Reusable modules like config.celery_app should not mutate process-global settings at import time
• Celery connection cleanup is still needed for gevent workers, but it must not affect the web pod import path

Closes PLA-1366