FIX: Use SolvedTopics to list posts in /activity/solved instead of user actions

[nattsw]

In #342 we moved solutions away from topic_custom_fields into proper tables, with the tables as the proper source of truth to a topic's solution.

The user's /my/activity/solved route uses user_actions which is not accurate, and a user has reported a bug where their solution is not reflected there (user actions are not a good representation of what a topic's solution is).

This PR introduces

• a new route to get solutions, and is mindful hide_user_profiles_from_public and such settings
• also mindful of PMs and private categories
• a new template that makes use of the <UserStream> to load posts safely and avoid reimplementation

After fix:

desc	📸
Regular post list in /activity
no solved (viewing another user)
no solved (viewing self)
one solved
solved with private categories
solved with hidden private categories
solved with expanded post	Screen.Recording.2025-06-27.at.12.21.23.PM.mov
solved with pagination	Screen.Recording.2025-06-26.at.5.34.33.PM.mov