Add Binance demo trading key settings and signed demo client

Store demo API key/secret encrypted in a single-row trading_account table,
expose get/put/delete/test endpoints under /api/settings/exchange, and add
a settings card to enter and test the key. The client is hard-wired to
demo-fapi.binance.com.

backend/src/alphapulse/models/trading.py

@@ -0,0 +1,24 @@
+from datetime import datetime
+
+from sqlalchemy import Boolean, CheckConstraint, DateTime, ForeignKey, LargeBinary, String, func
+from sqlalchemy.orm import Mapped, mapped_column
+
+from alphapulse.core.db import Base
+
+
+class TradingAccount(Base):
+    """币安模拟盘账户：单行（id=1）。key 只以密文存储；running 是引擎启停的唯一来源"""
+
+    __tablename__ = "trading_account"
+    __table_args__ = (CheckConstraint("id = 1", name="trading_account_single_row"),)
+
+    id: Mapped[int] = mapped_column(primary_key=True, default=1)
+    api_key_encrypted: Mapped[bytes | None] = mapped_column(LargeBinary)
+    api_secret_encrypted: Mapped[bytes | None] = mapped_column(LargeBinary)
+    api_key_hint: Mapped[str | None] = mapped_column(String(8))
+    running: Mapped[bool] = mapped_column(Boolean, default=False)
+    running_changed_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
+    # 交易分析用的模型配置；为空时用默认配置
+    provider_id: Mapped[int | None] = mapped_column(ForeignKey("ai_providers.id", ondelete="SET NULL"))
+    reasoning_effort: Mapped[str | None] = mapped_column(String(8))
+    updated_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now())