T2: Pydantic v2 — replace @dataclass with BaseModel
- All domain models now use pydantic.BaseModel
- Removed manual to_dict()/from_dict() — use model_dump(mode='json')/model_validate()
- Added field validators:
- Account.balance >= 0
- Transaction.amount != 0
- RecurringCashflow.amount != 0, frequency in {daily, weekly, monthly, yearly}
- Liability.interest, payment >= 0
- ExchangeRate.rate > 0
- ForecastScenario.multipliers >= 0
- pydantic v2 native UUID handling (auto str in JSON)
- Updated excel_sync.py to use model_validate()
- Updated tests/test_model.py and tests/test_currency.py
- Added pydantic>=2.0 to pyproject.toml
Tests: 63/63 pass.