Size demo trading by a configured capital instead of full account equity

Add trading_account.capital_usdt. Position sizing will use
min(capital, account equity); starting requires the capital to be set.
The trading page shows and edits it.

backend/alembic/versions/20261015_c3f19d7a2e58_trading_capital.py

@@ -0,0 +1,29 @@
+"""trading capital
+
+Revision ID: c3f19d7a2e58
+Revises: a54081406f23
+Create Date: 2026-10-15 10:10:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'c3f19d7a2e58'
+down_revision: Union[str, Sequence[str], None] = 'a54081406f23'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    # 用户要求只用设定的资金操盘，不动用账户全部余额（2026-10-15）
+    op.add_column('trading_account', sa.Column('capital_usdt', sa.Numeric(), nullable=True))
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    op.drop_column('trading_account', 'capital_usdt')