feat: add registered user stats to ActivityStats via pseudo_user

[Copilot]

The /activity-stats endpoint had no way to count newly registered users per day/week/month — only active users. Counting directly from base_user alongside pseudo_user active-user stats would cause double-counting for registered users.

Changes

• models/user.rs — New RegisteredUserStats struct (todays_registered, this_week_registered, this_month_registered); added registered: RegisteredUserStats field to ActivityStats
• db/user.rs — New concurrent registered_fut query using pseudo_user JOIN base_user filtered by base_user.created_at; result joined into the existing tokio::join! block
• .sqlx/ — Added offline query cache file for the new query

How double-counting is avoided

Each registered user has exactly one pseudo_user row (with base_user_id set). By driving the count from pseudo_user side via JOIN, every registered user appears exactly once regardless of what else queries base_user:

SELECT
    COUNT(*) FILTER (WHERE b.created_at >= date_trunc('month', CURRENT_DATE)) AS "this_month_registered!",
    COUNT(*) FILTER (WHERE b.created_at >= date_trunc('week',  CURRENT_DATE)) AS "this_week_registered!",
    COUNT(*) FILTER (WHERE b.created_at >= CURRENT_DATE)                      AS "todays_registered!"
FROM pseudo_user p
JOIN base_user b ON b.id = p.base_user_id

The existing RecentUserStats (active users by last_active) is unchanged — it already only touches pseudo_user.