Risk controls, alerts, API token, more exchanges, presets

- RiskManager halts + flattens on max daily loss / drawdown, caps position
  size and trades per day; adds a manual kill switch (POST /api/bot/kill).
- Telegram and webhook notifications on trades, errors and halts.
- Optional API bearer token on /api, injected server-side by the frontend
  nginx so the browser never sees it.
- Kraken and Coinbase market data (public); Coinbase live broker. Live on an
  unsupported exchange now errors instead of silently falling back to paper.
- Strategy presets backed by the StrategyConfig table (CRUD API + UI).
- Idempotent additive-column migration so existing SQLite databases upgrade in
  place (fixes the new strategy_configs.name column).

backend/app/models.py

@@ -45,6 +45,7 @@ class StrategyConfig(Base):
     __tablename__ = "strategy_configs"
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True)
+    name: Mapped[str] = mapped_column(String(80), default="")
     strategy_key: Mapped[str] = mapped_column(String(64), index=True)
     symbol: Mapped[str] = mapped_column(String(32))
     timeframe: Mapped[str] = mapped_column(String(16))