Per-stat delta over the longest window with available history.

Reads wallet_pnl_snapshots (daily 02:30 UTC). For each metric:

• if a snapshot 7 days ago exists -> return (delta_value, window: "7d")
• else if 1 day ago exists -> return (delta_value, window: "24h")
• else null (no baseline yet, e.g. fresh fields just added today)

Stats: total_pnl, closed_count, profit_factor, win_rate.

Powers small sub-row chips under each stat card on the wallet page. Cache 5min — snapshots change once a day so this is plenty.