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_model.py

@@ -21,8 +21,8 @@ class TestAccount:
 
     def test_to_dict_roundtrip(self):
         a = Account(name="Test", balance=100.0)
-        d = a.to_dict()
-        a2 = Account.from_dict(d)
+        d = a.model_dump(mode="json")
+        a2 = Account.model_validate(d)
         assert a2.name == a.name
         assert a2.balance == a.balance
         assert a2.currency == a.currency
@@ -35,8 +35,8 @@ class TestTransaction:
 
     def test_roundtrip(self):
         t = Transaction(amount=-100.0, category="rent", description="test")
-        d = t.to_dict()
-        t2 = Transaction.from_dict(d)
+        d = t.model_dump(mode="json")
+        t2 = Transaction.model_validate(d)
         assert t2.amount == t.amount
         assert t2.category == t.category
         assert t2.description == t.description