Refactor user growth chart data model and enhance demo user creation

- Renamed `totalUsers` and `activeUsers` to `total_users` and `active_users` across frontend and backend for consistency.
- Enhanced demo user creation by randomizing `created_at` dates for realistic charts.
- Expanded demo data to include `is_active` for demo users, improving user status representation.
- Refined admin dashboard statistics to support updated user growth data model.

backend/app/schemas/users.py

@@ -23,6 +23,7 @@ class UserBase(BaseModel):
 class UserCreate(UserBase):
     password: str
     is_superuser: bool = False
+    is_active: bool = True
 
     @field_validator("password")
     @classmethod