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.

domain/asset.py

@@ -1,27 +1,10 @@
-from dataclasses import dataclass, field
 from uuid import UUID, uuid4
 
+from pydantic import BaseModel, Field
 
-@dataclass
-class Asset:
-    id: UUID = field(default_factory=uuid4)
+
+class Asset(BaseModel):
+    id: UUID = Field(default_factory=uuid4)
     name: str = ""
     value: float = 0.0
     growth_rate: float = 0.0
-
-    def to_dict(self) -> dict:
-        return {
-            "id": str(self.id),
-            "name": self.name,
-            "value": self.value,
-            "growth_rate": self.growth_rate,
-        }
-
-    @classmethod
-    def from_dict(cls, data: dict) -> "Asset":
-        return cls(
-            id=UUID(data["id"]),
-            name=data["name"],
-            value=data.get("value", 0.0),
-            growth_rate=data.get("growth_rate", 0.0),
-        )