T1: restructure to domain/application/infrastructure + schema version 1

- Move cashflow_model/ -> domain/
- Move engine/ -> application/
- Move cli/, sync/, ai/ -> infrastructure/
- Update all imports across codebase and tests
- Add version=1 to FinancialModel.to_dict() with migration hook in from_dict
- Update pyproject.toml: cf entry point and packages.find

Tests: 63/63 pass.

domain/liability.py

@@ -0,0 +1,30 @@
+from dataclasses import dataclass, field
+from uuid import UUID, uuid4
+
+
+@dataclass
+class Liability:
+    id: UUID = field(default_factory=uuid4)
+    name: str = ""
+    balance: float = 0.0
+    interest: float = 0.0
+    payment: float = 0.0
+
+    def to_dict(self) -> dict:
+        return {
+            "id": str(self.id),
+            "name": self.name,
+            "balance": self.balance,
+            "interest": self.interest,
+            "payment": self.payment,
+        }
+
+    @classmethod
+    def from_dict(cls, data: dict) -> "Liability":
+        return cls(
+            id=UUID(data["id"]),
+            name=data["name"],
+            balance=data.get("balance", 0.0),
+            interest=data.get("interest", 0.0),
+            payment=data.get("payment", 0.0),
+        )