[feature] Weekly leaderboard view

[EvanPaules]

The all-time board stratifies as early players pile up XP. A weekly view keeps it competitive.

Idea

claudexp board --weekly shows the top XP gained in the last 7 days, resetting on Monday.

Implementation paths

Two options, pick one in the PR:

MVP (local only): the local ~/.claudexp/data.db already tracks every session with a timestamp. Sum sessions where ended_at >= now() - 7 days per user. Only works for your own row, but is zero-backend and ships in ~20 LOC.

Full (cloud): append-only session_events table on Supabase with (username, xp_delta, at) rows, written by hook.js after the scored total is known. The weekly board is a single query: select username, sum(xp_delta) from session_events where at >= now() - interval '7 days' group by username order by sum desc. Requires RLS: anyone can insert where username = their own + owner token matches; anyone can select all.

Open questions

• Week boundary: UTC Mon-Sun vs local time?
• Also add --monthly? (trivial if weekly works)
• If we go full-cloud, what's the retention policy on session_events? (e.g., delete rows older than 60 days to keep the table tiny)