fix(digest): send group id properly

[raquelmsmith]

Problem

Customer.io rejected our periodic digest events because the group ID was missing.

Changes

Makes sure there is an organization ID on the event groups

👉 Stay up-to-date with PostHog coding conventions for a smoother review.

Does this work well for both Cloud and self-hosted?

How did you test this code?

added a test

[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: posthog/tasks/usage_report.py

Function	Unhandled Issue
capture_event	AssertionError: group_key must have (<class 'numbers.Number'>, (<class 'str'>,), <class 'uuid.UUID'>), got: None ... Event Count: 5.3k

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