add paper trading: mark a setup as taken, track open P&L, sell

New paper_trades table (migration 007) + service/router. "Mark as taken" on each
setup card (shares prefilled from position sizing, entry from current price, both
editable) records a simulated trade. Overview gains an Open Trades table that
marks each position to the latest close — P&L in $, %, and R-multiples — with a
total unrealized P&L footer and a Sell button to close at the current price.
Closed trades are retained for future realized-P&L reporting.

Deploy: alembic upgrade (new paper_trades table).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/models/__init__.py

@@ -9,6 +9,7 @@ from app.models.trade_setup import TradeSetup
 from app.models.watchlist import WatchlistEntry
 from app.models.settings import SystemSetting, IngestionProgress
 from app.models.alert import AlertLog
+from app.models.paper_trade import PaperTrade
 
 __all__ = [
     "Ticker",
@@ -24,4 +25,5 @@ __all__ = [
     "SystemSetting",
     "IngestionProgress",
     "AlertLog",
+    "PaperTrade",
 ]