fix(admin): link the member for ban and member-note activities

[mroderick]

Problem

Admin member profiles return 500 when the activity log contains a ban or member-note entry. The view builds a polymorphic [:admin, trackable] link, and neither Ban nor MemberNote has an admin show route (bans are a write-only resource nested under members; member notes only have a create route), so rendering raises undefined method 'admin_ban_path'.

Change

• activity_subject maps Ban and MemberNote trackables to their member, so the log links to admin_member_path instead of a route that does not exist.
• Regression specs cover both the helper mapping and the rendered profile page.

Review notes

• Look first at the mapping choice: link the banned/noted member rather than dropping the link. Dropping it (returning nil) also stops the crash but loses the "for whom" context in the log line.
• Deliberately not done: no show routes added for bans or notes (no such pages exist), and no generic route-existence guard for trackable types added in the future.

Failure detail

The activity log links subjects with link_to(label, [:admin, subject]). For a Ban trackable that resolves to admin_ban_path, which is undefined; the template error propagates as a 500 for the whole profile page. Any member with a ban activity they performed (the log lists activities by owner) is affected. MemberNote has the same shape and would fail identically.