Backend/moderation: add from_visibility filter to SetUserContentVisibility

[aapeliv]

Adds an optional from_visibility repeated field to SetUserContentVisibilityReq that narrows the bulk sweep to moderation states currently at one of the given visibilities. An empty list preserves the existing behavior of sweeping any visibility.

This lets admins do targeted transitions (e.g. flip only currently-visible content to shadowed) without touching content that's already been moderated to a different state.

Testing

Explain how you tested this PR and give clear steps so the reviewer can replicate.

• Added unit tests covering: filter that matches nothing, filter that matches one, multi-visibility filter, empty filter (any), and rejection of MODERATION_VISIBILITY_UNSPECIFIED in the filter list.
• make format, make mypy, and uv run pytest src/tests/test_moderation.py all pass locally.

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.

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Actions	Updated (UTC)
couchers	Ready	Preview	May 7, 2026 11:54pm

[github-actions]

📝 Release Notes

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

Reason: This change adds a targeted moderation API filter for admins to bulk-change content visibility based on current visibility state. It is an internal backend moderation tool, not something regular end users would notice, and it does not represent a major user-facing feature or broadly significant infrastructure improvement.

🤖 Bot Debug Information

Model: couchers.openai.gpt-5.4
Decision: exclude
Reasoning: This change adds a targeted moderation API filter for admins to bulk-change content visibility based on current visibility state. It is an internal backend moderation tool, not something regular end users would notice, and it does not represent a major user-facing feature or broadly significant infrastructure improvement.