Backend: Add ListAdminActions and ListModerationStates RPCs

[aapeliv]

Adds two new admin/moderation review RPCs to support browsing historical activity from the admin UI:

• Admin.ListAdminActions — paginated list of admin action log entries, optionally filtered by admin_user_id and/or target_user_id. Returns newest first. The AdminActionLog message gains target_user_id and target_username fields so callers can render rows without an extra lookup.
• Moderation.ListModerationStates — paginated list of ModerationState rows, ordered chronologically (with optional newest_first). Supports filtering to states whose underlying content was authored by a specific user, by EXISTS-checking each registered moderation object type's author column.

Testing

• Added unit tests covering empty results, ordering, filtering by admin/target/author, and pagination for both RPCs.
• make format and make mypy pass.
• uv run pytest src/tests/test_admin.py src/tests/test_moderation.py (run locally before pushing).

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

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)
couchers	Ready	Preview	May 7, 2026 2:54am

[github-actions]

📝 Release Notes

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

Reason: This PR adds backend RPCs to support admin and moderation teams browsing historical admin actions and moderation states in internal admin UI. It is not a user-facing feature for end users, and while moderation-related, it appears to be internal tooling rather than a safety feature users directly notice. It also does not rise to the level of major technical infrastructure work for release notes.

🤖 Bot Debug Information

Model: couchers.openai.gpt-5.4
Decision: exclude
Reasoning: This PR adds backend RPCs to support admin and moderation teams browsing historical admin actions and moderation states in internal admin UI. It is not a user-facing feature for end users, and while moderation-related, it appears to be internal tooling rather than a safety feature users directly notice. It also does not rise to the level of major technical infrastructure work for release notes.