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.

tests/test_model.py

@@ -1,6 +1,6 @@
 from pathlib import Path
 
-from cashflow_model import (
+from domain import (
     Account,
     Asset,
     ExchangeRate,