Scaffold project layout, DB schema and settings

Package skeleton, pyproject/alembic config, env-driven settings
(app/config.py), and the SQLAlchemy models + initial Alembic migration
covering users, UTXO events, rounds/participants, round config,
pending transactions, withdrawals and the audit log.

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

app/db/base.py

@@ -0,0 +1,11 @@
+from sqlalchemy.ext.asyncio import async_sessionmaker, create_async_engine
+from sqlalchemy.orm import DeclarativeBase
+
+from app.config import settings
+
+engine = create_async_engine(settings.database_url)
+AsyncSessionLocal = async_sessionmaker(engine, expire_on_commit=False)
+
+
+class Base(DeclarativeBase):
+    pass