Return profiles matching prefix for @mention autocomplete.

Ranking:

1. workspace_member — the workspace's people: owner, direct members, the members of the team the workspace is linked to (team_id), and the caller. The same union as GET /v2/workspaces/{id}/members; a team member has access to the workspace, so they are a person in it (2026-09-09: a confirmed team member could not be tagged from a workspace their team owns). Matched on handle, display name OR email, so a member with no handle is found by name and a member with no name yet is found by email. With an empty prefix this is the whole list, which is what a bare "@" shows. Sorted by handle/name.
2. recent_collaborator — profiles whose handle starts with prefix and who share a workspace with the caller where activity occurred within the last 90 days. Sorted by last_collaborated_at desc, then alphabetical by handle. Skipped for an empty prefix.

The caller is included. A solo workspace has exactly one person in it, and that person could not tag themself (2026-09-05: "No matching profiles" on the owner's own workspace).

Recency gate ("90-day window"): if no shared workspace has any activity (content, launch, or workspace-scope comment) within the last 90 days, the candidate is dropped. v0 simplification documented in _latest_workspace_activity.