Refactor code structure for improved readability and maintainability

app/models/plan.py

@@ -0,0 +1,21 @@
+from typing import TYPE_CHECKING
+
+from sqlmodel import Field, Relationship
+
+from app.models.base import BaseModel
+
+if TYPE_CHECKING:
+    from app.models.user import User
+
+
+class Plan(BaseModel, table=True):
+    """Database model for a plan."""
+
+    code: str = Field(index=True, unique=True, nullable=False)
+    name: str = Field(nullable=False)
+    description: str | None = Field(default=None)
+    credits: int = Field(default=0, ge=0, nullable=False)
+    max_credits: int = Field(default=0, ge=0, nullable=False)
+
+    # relationships
+    users: list["User"] = Relationship(back_populates="plan")