T1: restructure to domain/application/infrastructure + schema version 1

- Move cashflow_model/ -> domain/
- Move engine/ -> application/
- Move cli/, sync/, ai/ -> infrastructure/
- Update all imports across codebase and tests
- Add version=1 to FinancialModel.to_dict() with migration hook in from_dict
- Update pyproject.toml: cf entry point and packages.find

Tests: 63/63 pass.

domain/__init__.py

@@ -0,0 +1,22 @@
+from domain.account import Account
+from domain.asset import Asset
+from domain.currency import CURRENCY_SYMBOLS, CurrencyConverter, CurrencyError, ExchangeRate
+from domain.liability import Liability
+from domain.model import FinancialModel
+from domain.recurring import RecurringCashflow
+from domain.scenario import ForecastScenario
+from domain.transaction import Transaction
+
+__all__ = [
+    "Account",
+    "Transaction",
+    "RecurringCashflow",
+    "Asset",
+    "Liability",
+    "ForecastScenario",
+    "FinancialModel",
+    "ExchangeRate",
+    "CurrencyConverter",
+    "CurrencyError",
+    "CURRENCY_SYMBOLS",
+]