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.

tests/test_currency.py

@@ -6,8 +6,8 @@ from domain import CurrencyConverter, CurrencyError, ExchangeRate
 class TestExchangeRate:
     def test_to_dict_roundtrip(self):
         rate = ExchangeRate(from_currency="USD", to_currency="RUB", rate=80.0)
-        d = rate.to_dict()
-        r2 = ExchangeRate.from_dict(d)
+        d = rate.model_dump(mode="json")
+        r2 = ExchangeRate.model_validate(d)
         assert r2.from_currency == "USD"
         assert r2.to_currency == "RUB"
         assert r2.rate == 80.0