Discover moderated content models dynamically

[aapeliv]

Pure refactor of the Unified Moderation System plumbing, no behavior change.

Previously the set of UMS-governed models was encoded in several hand-maintained places: the seven per-object-type branches inside moderation_state_column_visible, the moderationobjecttype2model dict in servicers/moderation.py, and the _ModeratedContent union in sql.py. Adding a moderated type meant updating each.

This replaces them with a single source of truth on the models themselves:

• Each moderated model declares __moderation_object_type__ alongside the existing __moderation_author_column__.
• get_moderated_models() discovers them by scanning the SQLAlchemy mapper registry and returns {ModerationObjectType: ModeratedModel}, where ModeratedModel resolves the model, object type, and author column.
• moderation_state_column_visible and servicers/moderation.py now iterate that registry instead of hardcoded lists.
• The duplicated mutual-block exists(...) clause is factored into a _users_block_each_other helper.

Testing

• make format and make mypy pass (the two remaining mypy errors — test_calendar_events.py missing ics stubs and fixtures/misc.py return type — are pre-existing on develop).
• 142 tests pass across test_moderation, test_threads, test_notifications, test_discussions.
• To replicate: cd app/backend && make format && make mypy && uv run pytest.

Backend checklist

• Added tests for any new code or added a regression test if fixing a bug
• Run the backend locally and it works
• Added migrations if there are any database changes, rebased onto develop if necessary for linear migration history (no DB changes)

For maintainers

• Maintainers can push commits to my branch
• Maintainers can merge this PR for me

This PR was created with the Couchers PR skill.

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Project	Deployment	Actions	Updated (UTC)
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[tristanlabelle]

Nice, much better. I think we can still go one step further with typing.Protocol.

[tristanlabelle]

Have you tried using typing.Protocol? My proof-of-concept works great and avoids the union and getattr(cls, cls.__moderation_author_column__),.

from enum import Enum, auto
from typing import ClassVar, Protocol

class ModerationObjectType(Enum):
    event_occurrence = auto()

class ModeratedContent(Protocol):
    __moderation_author_column__: ClassVar[str]
    __moderation_object_type__: ClassVar[ModerationObjectType]
    # id: Mapped[int]

class EventOccurrence:
    __moderation_author_column__ = "creator_user_id"
    __moderation_object_type__ = ModerationObjectType.event_occurrence


moderated_content_type: type[ModeratedContent] = EventOccurrence
print(moderated_content_type.__moderation_author_column__)
print(moderated_content_type.__moderation_object_type__)

[aapeliv]

awesome, implemented, thanks! typing was a bit inflexible, otherwise worked

[github-actions]

📝 Release Notes

This PR does not need to be included in release notes.

Reason: This PR is explicitly described as a pure refactor of moderation system plumbing with no behavior change. It improves internal maintainability and reduces hand-maintained duplication, but does not add a user-facing feature, noticeable UX improvement, or clearly significant end-user-impacting reliability change on its own.

🤖 Bot Debug Information

Model: couchers.openai.gpt-5.4
Decision: exclude
Reasoning: This PR is explicitly described as a pure refactor of moderation system plumbing with no behavior change. It improves internal maintainability and reduces hand-maintained duplication, but does not add a user-facing feature, noticeable UX improvement, or clearly significant end-user-impacting reliability change on its own.