[background_tasks] No hook for a consumer module to register a beat schedule; docs imply one, and the commonly-cited on_after_configure signal doesn't fire under build_celery #199

Summary

A consumer module can autodiscover tasks (ship tasks.py) but cannot register a periodic/beat schedule through any framework hook, and the obvious Celery approach is broken in this setup.

Environment

• Framework checkout: v0.0.17-5-g2b079c9; smpy / published simple_module_* / @simple-module-py/*: 0.0.17
• Python 3.12.3, uv 0.11.7, node v24.15.0, Linux (Ubuntu)

Found while building a real consumer app (Invoice Manager) on the framework.

Details

• docs/modules/background_tasks.md implies make beat runs "beat schedules other modules add", but beat_schedule is a hardcoded dict in build_celery (celery_app.py) with the two internal tasks and no consumer registry/hook. No module adds entries.
• The commonly-cited from celery.signals import on_after_configure does not work: on_after_configure is an app-instance signal (not a member of celery.signals → ImportError), and build_celery calls conf.update(...) (which fires it) before autodiscover_tasks(...) imports the module's tasks.py — so even connecting to the right object misses the already-fired signal.

Working pattern (used as the workaround)

In the module's tasks.py:

from celery import current_app, shared_task
from celery.schedules import crontab

@shared_task(name="invoices.generate_recurring")
def generate_recurring() -> int: ...

@current_app.on_after_finalize.connect
def _register(sender, **kw):
    sender.add_periodic_task(crontab(hour=6, minute=0), generate_recurring.s(),
                             name="invoices-generate-recurring-daily")

Verified: the entry lands in beat_schedule (0 6 * * *) alongside the built-ins.

Impact

Recurring features need this. Suggest either a first-class "register periodic task" hook for modules, or documenting the on_after_finalize pattern in background_tasks.md (and not on_after_configure).