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/alembic/versions/20261015_a7d3e9c41f02_trading_account.py

@@ -0,0 +1,41 @@
+"""trading account
+
+Revision ID: a7d3e9c41f02
+Revises: 5c2e8f1a7b3d
+Create Date: 2026-10-15 10:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'a7d3e9c41f02'
+down_revision: Union[str, Sequence[str], None] = '5c2e8f1a7b3d'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    op.create_table('trading_account',
+    sa.Column('id', sa.Integer(), autoincrement=False, nullable=False),
+    sa.Column('api_key_encrypted', sa.LargeBinary(), nullable=True),
+    sa.Column('api_secret_encrypted', sa.LargeBinary(), nullable=True),
+    sa.Column('api_key_hint', sa.String(length=8), nullable=True),
+    sa.Column('running', sa.Boolean(), nullable=False),
+    sa.Column('running_changed_at', sa.DateTime(timezone=True), nullable=True),
+    sa.Column('provider_id', sa.Integer(), nullable=True),
+    sa.Column('reasoning_effort', sa.String(length=8), nullable=True),
+    sa.Column('updated_at', sa.DateTime(timezone=True), server_default=sa.text('now()'), nullable=False),
+    sa.CheckConstraint('id = 1', name='trading_account_single_row'),
+    sa.ForeignKeyConstraint(['provider_id'], ['ai_providers.id'], ondelete='SET NULL'),
+    sa.PrimaryKeyConstraint('id')
+    )
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    op.drop_table('trading_account')