Run demo trading and allocate capital per symbol

Move running, running_changed_at and capital_usdt from the account to
each trading symbol; the migration copies the account capital to every
symbol and leaves them all stopped. Each symbol is sized against its
own capital with a 3x notional cap, and the capitals together must not
exceed account equity. Scheduling, dry-run decisions and the executor
follow the per-symbol flag. The global start/stop button is replaced by
start/stop and capital controls on each symbol's plan card.

backend/alembic/versions/20261016_a7d3e5f20c18_trading_per_symbol_engine.py

@@ -0,0 +1,41 @@
+"""trading per symbol engine
+
+Revision ID: a7d3e5f20c18
+Revises: f2a6c8d91b47
+Create Date: 2026-10-16 15:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'a7d3e5f20c18'
+down_revision: Union[str, Sequence[str], None] = 'f2a6c8d91b47'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    # 用户决定启停和交易资金按币种分开（2026-10-16）：原账户资金复制给每个币，全部停止
+    op.add_column('trading_symbols', sa.Column('running', sa.Boolean(), nullable=False, server_default=sa.false()))
+    op.add_column('trading_symbols', sa.Column('running_changed_at', sa.DateTime(timezone=True), nullable=True))
+    op.add_column('trading_symbols', sa.Column('capital_usdt', sa.Numeric(), nullable=True))
+    op.execute("UPDATE trading_symbols SET capital_usdt = (SELECT capital_usdt FROM trading_account WHERE id = 1)")
+    op.drop_column('trading_account', 'running')
+    op.drop_column('trading_account', 'running_changed_at')
+    op.drop_column('trading_account', 'capital_usdt')
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    op.add_column('trading_account', sa.Column('capital_usdt', sa.Numeric(), nullable=True))
+    op.add_column('trading_account', sa.Column('running_changed_at', sa.DateTime(timezone=True), nullable=True))
+    op.add_column('trading_account', sa.Column('running', sa.Boolean(), nullable=False, server_default=sa.false()))
+    op.execute("UPDATE trading_account SET capital_usdt = (SELECT max(capital_usdt) FROM trading_symbols)")
+    op.drop_column('trading_symbols', 'capital_usdt')
+    op.drop_column('trading_symbols', 'running_changed_at')
+    op.drop_column('trading_symbols', 'running')