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>

.gitignore

@@ -0,0 +1,8 @@
+.venv/
+__pycache__/
+*.pyc
+.env
+*.db
+master.xprv.enc
+.pytest_cache/
+*.egg-info/