Add default USD-M perpetual BTC/ETH/BNB and fix futures WS stream

Seed um BTCUSDT/ETHUSDT/BNBUSDT as default symbols. The legacy futures
route fstream.binance.com/stream accepts connections but no longer pushes
data, so switch to /market/stream. Receive with a timeout so symbol
changes are picked up and silent connections are reconnected.

backend/alembic/versions/20261013_bf0ffbfcaf9a_default_um_symbols.py

@@ -0,0 +1,35 @@
+"""default um symbols
+
+Revision ID: bf0ffbfcaf9a
+Revises: c7e1beb72c75
+Create Date: 2026-10-13 06:01:27.204671
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'bf0ffbfcaf9a'
+down_revision: Union[str, Sequence[str], None] = 'c7e1beb72c75'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    # U 本位永续的默认交易对；精度取自 2026-10-13 币安 fapi exchangeInfo（tickSize 0.10/0.01/0.010，stepSize 0.001/0.001/0.01）
+    op.execute(sa.text("""
+        INSERT INTO market_symbols (market, symbol, base, quote, price_precision, volume_precision, status, is_default)
+        VALUES ('um', 'BTCUSDT', 'BTC', 'USDT', 1, 3, 'pending', true),
+               ('um', 'ETHUSDT', 'ETH', 'USDT', 2, 3, 'pending', true),
+               ('um', 'BNBUSDT', 'BNB', 'USDT', 2, 2, 'pending', true)
+        ON CONFLICT (market, symbol) DO UPDATE SET is_default = true
+    """))
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    op.execute(sa.text("UPDATE market_symbols SET is_default = false WHERE market = 'um' AND symbol IN ('BTCUSDT', 'ETHUSDT', 'BNBUSDT')"))