fix: add pnl_pct and capital_invested migration for trade_entry_prices

Both columns were referenced in queries (reliability, Kelly, calibration,
backtest) but missing from the ALTER TABLE migration block, causing
sqlite3.OperationalError: no such column: pnl_pct on existing VPS DBs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/services/database.py

@@ -205,6 +205,8 @@ def init_db():
         ("trade_score", "REAL"),
         ("matched_profile", "TEXT"),
         ("last_seen_at", "TEXT"),
+        ("pnl_pct", "REAL"),
+        ("capital_invested", "REAL"),
     ]:
         try:
             c.execute(f"ALTER TABLE trade_entry_prices ADD COLUMN {col} {definition}")