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/model.py

@@ -1,7 +1,8 @@
 import json
-from dataclasses import dataclass, field
 from pathlib import Path
 
+from pydantic import BaseModel, Field
+
 from domain.account import Account
 from domain.asset import Asset
 from domain.currency import DEFAULT_RATES, ExchangeRate
@@ -11,37 +12,41 @@ from domain.scenario import ForecastScenario
 from domain.transaction import Transaction
 
 
-@dataclass
-class FinancialModel:
-    base_currency: str = "RUB"
-    accounts: list[Account] = field(default_factory=list)
-    transactions: list[Transaction] = field(default_factory=list)
-    recurring: list[RecurringCashflow] = field(default_factory=list)
-    assets: list[Asset] = field(default_factory=list)
-    liabilities: list[Liability] = field(default_factory=list)
-    scenarios: list[ForecastScenario] = field(default_factory=list)
-    exchange_rates: list[ExchangeRate] = field(default_factory=lambda: DEFAULT_RATES.copy())
+class FinancialModel(BaseModel):
+    """Корневая модель финансового плана."""
 
-    SCHEMA_VERSION = 1
+    SCHEMA_VERSION: int = 1  # NB: не Field — это class-level metadata, не pydantic field
+
+    base_currency: str = "RUB"
+    accounts: list[Account] = Field(default_factory=list)
+    transactions: list[Transaction] = Field(default_factory=list)
+    recurring: list[RecurringCashflow] = Field(default_factory=list)
+    assets: list[Asset] = Field(default_factory=list)
+    liabilities: list[Liability] = Field(default_factory=list)
+    scenarios: list[ForecastScenario] = Field(default_factory=list)
+    exchange_rates: list[ExchangeRate] = Field(
+        default_factory=lambda: [r.model_copy() for r in DEFAULT_RATES]
+    )
 
     def to_dict(self) -> dict:
+        """Сериализация в dict с version и UUID-as-str (для JSON)."""
         return {
             "version": self.SCHEMA_VERSION,
             "base_currency": self.base_currency,
-            "accounts": [a.to_dict() for a in self.accounts],
-            "transactions": [t.to_dict() for t in self.transactions],
-            "recurring": [r.to_dict() for r in self.recurring],
-            "assets": [a.to_dict() for a in self.assets],
-            "liabilities": [li.to_dict() for li in self.liabilities],
-            "scenarios": [s.to_dict() for s in self.scenarios],
-            "exchange_rates": [r.to_dict() for r in self.exchange_rates],
+            "accounts": [a.model_dump(mode="json") for a in self.accounts],
+            "transactions": [t.model_dump(mode="json") for t in self.transactions],
+            "recurring": [r.model_dump(mode="json") for r in self.recurring],
+            "assets": [a.model_dump(mode="json") for a in self.assets],
+            "liabilities": [li.model_dump(mode="json") for li in self.liabilities],
+            "scenarios": [s.model_dump(mode="json") for s in self.scenarios],
+            "exchange_rates": [e.model_dump(mode="json") for e in self.exchange_rates],
         }
 
     @classmethod
     def from_dict(cls, data: dict) -> "FinancialModel":
         version = data.get("version", 0)
         if version == 0:
-            return cls._from_v0(data)
+            return cls._from_legacy(data)
         if version == 1:
             return cls._from_v1(data)
         raise ValueError(f"Unsupported FinancialModel version: {version}")
@@ -50,18 +55,18 @@ class FinancialModel:
     def _from_v1(cls, data: dict) -> "FinancialModel":
         return cls(
             base_currency=data.get("base_currency", "RUB"),
-            accounts=[Account.from_dict(a) for a in data.get("accounts", [])],
-            transactions=[Transaction.from_dict(t) for t in data.get("transactions", [])],
-            recurring=[RecurringCashflow.from_dict(r) for r in data.get("recurring", [])],
-            assets=[Asset.from_dict(a) for a in data.get("assets", [])],
-            liabilities=[Liability.from_dict(li) for li in data.get("liabilities", [])],
-            scenarios=[ForecastScenario.from_dict(s) for s in data.get("scenarios", [])],
-            exchange_rates=[ExchangeRate.from_dict(r) for r in data.get("exchange_rates", [])],
+            accounts=[Account.model_validate(a) for a in data.get("accounts", [])],
+            transactions=[Transaction.model_validate(t) for t in data.get("transactions", [])],
+            recurring=[RecurringCashflow.model_validate(r) for r in data.get("recurring", [])],
+            assets=[Asset.model_validate(a) for a in data.get("assets", [])],
+            liabilities=[Liability.model_validate(li) for li in data.get("liabilities", [])],
+            scenarios=[ForecastScenario.model_validate(s) for s in data.get("scenarios", [])],
+            exchange_rates=[ExchangeRate.model_validate(r) for r in data.get("exchange_rates", [])],
         )
 
     @classmethod
-    def _from_v0(cls, data: dict) -> "FinancialModel":
-        # Legacy: файлы, сохранённые до введения version. Совпадает по структуре с v1.
+    def _from_legacy(cls, data: dict) -> "FinancialModel":
+        # Legacy: файлы без version. Структура совпадает с v1.
         return cls._from_v1(data)
 
     def save(self, path: str | Path) -> None: