Fix survivorship-biased win rate; add backtest, per-position cap, Discord alerts

The scanner measured win rate over /closed-positions only, but Polymarket
only redeems winning shares — losers sit unredeemed in /positions at
curPrice 0 and never enter closed-positions. That made win rates wildly
inflated (e.g. 90.6% vs a true 48.3%). Win rate now unions both endpoints
over a 90-day window. With the honest metric, ~no top wallet exceeds ~60%;
true rates cluster near 50%.

Also:
- backtest.py: replay a watchlist over a recent window, fill at historical
  price, mark outcomes from resolution. A 7d run of 4 top wallets returned
  -48%, confirming flat-size entry-copying is -EV at ~50% hit rates.
- copytrade.py: add max_position_usd cap (proportional adds could otherwise
  balloon one position to the whole exposure limit) and Discord webhook
  alerts on every would-be trade.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitignore

@@ -5,4 +5,5 @@ __pycache__/
 # never commit live credentials or runtime state
 config.json
 copytrade_state.json
+*.log
 *.tmp