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>

.env.example

@@ -0,0 +1,20 @@
+DATABASE_URL=sqlite+aiosqlite:///./plm_lottery.db
+
+ELECTRUM_HOST=santantonio.sytes.net
+ELECTRUM_PORT=50002
+ELECTRUM_USE_SSL=true
+
+# Fernet key protecting the master xprv at rest. Generate with:
+#   python -c "from cryptography.fernet import Fernet; print(Fernet.generate_key().decode())"
+XPRV_ENCRYPTION_KEY=
+MASTER_KEY_PATH=./master.xprv.enc
+
+# Random secret for JWT session signing. Generate with:
+#   python -c "import secrets; print(secrets.token_urlsafe(32))"
+JWT_SECRET=
+
+# Bearer token required on the admin endpoints (X-Admin-Token header). Generate with:
+#   python -c "import secrets; print(secrets.token_urlsafe(32))"
+ADMIN_TOKEN=
+
+ROUND_DURATION_SECONDS=600