Tie the withdrawal minimum to the bet amount instead of a separate field

RoundConfig.min_amount_sats was an independently-configurable floor that
could drift out of sync with bet_amount_sats for no real reason (deposits
never had a server-side minimum anyway). Drop the field and enforce
amount_sats >= config.bet_amount_sats directly in request_withdrawal.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

migrations/versions/6cb50b29f64c_drop_min_amount_sats.py

@@ -0,0 +1,31 @@
+"""drop min_amount_sats, withdrawal minimum now equals bet amount
+
+Revision ID: 6cb50b29f64c
+Revises: 5f2079b95b33
+Create Date: 2026-07-22 16:50:38.765233
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '6cb50b29f64c'
+down_revision: Union[str, Sequence[str], None] = '5f2079b95b33'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    with op.batch_alter_table('round_config') as batch_op:
+        batch_op.drop_column('min_amount_sats')
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    with op.batch_alter_table('round_config') as batch_op:
+        batch_op.add_column(sa.Column('min_amount_sats', sa.BigInteger(), nullable=False, server_default='100000000'))
+        batch_op.alter_column('min_amount_sats', server_default=None)