feat(gdd): Add GroupDerivedData.generated_at

[kcons]

Knowing when GroupDerivedData generation began allows to roughly track what log entries it had access to, and this in turn allows us to know if an incremental update is unsafe because it is based on an earlier regeneration run.

The plan is to use this for safe in-place regeneration.

[ceorourke]

isn't this still true?

[github-actions]

This PR has a migration; here is the generated SQL for src/sentry/migrations/1141_add_generated_at_to_group_derived_data.py

for 1141_add_generated_at_to_group_derived_data in sentry

--
-- Add field generated_at to groupderiveddata
--
ALTER TABLE "sentry_groupderiveddata" ADD COLUMN "generated_at" timestamp with time zone DEFAULT (STATEMENT_TIMESTAMP()) NOT NULL;

[sentry-warden]

models.functions.Now() raises AttributeError on module import

models.functions.Now() fails because django.db.models does not expose functions; every other db_default=Now() usage in the codebase explicitly imports Now from django.db.models.functions.

Evidence

• models is bound to django.db.models (line 4: from django.db import models).
• django.db.models does not re-export the functions submodule, so models.functions raises AttributeError at class definition time.
• Every other db_default=Now() usage in Sentry uses an explicit from django.db.models.functions import Now import (e.g., groupactionlogentry.py, authidentityreplica.py, outbox.py).
• Django's own migration autogenerator also emits import django.db.models.functions.datetime rather than models.functions.Now().

_{Identified by Warden sentry-backend-bugs · XYU-67D}