Diff summary of wallet activity since since timestamp.

Powers the 'What changed since last visit' banner shown to returning users. Returns trades count, P&L delta, and the single most recent trade for context.

• trades_count: real-time, sourced from trades_apr2026_plus (indexer).
• pnl_delta_usd: closed_positions_cache (may be slightly stale; lazy-refreshed by /overview).
• last_move: most recent trade by this wallet in the window, with market title.
• since is capped at NOW - 30d (banner stops being useful past that horizon).
• If since is < 60s ago, returns trades_count=0 ("nothing changed since you blinked").