Create and add schemas in new backend file under the root directory

[KailuanLiu]

Add schemas as a first step to setup our backend

• userSchema.jsx
  • username: string
  • email: string
  • password: hashed string
  • profile_picture: URL or image path (optional to skip for now)
  • created_at: timestamp
  • updated_at: timestamp
• itemSchema.jsx
  • title: string
  • description: string
  • due_date: timestamp
  • completed: boolean
  • priority: (enum: 'low', 'medium', 'high')
  • created_at: timestamp
  • updated_at: timestamp
  • user_id: optional multi-user
• labelSchema.jsx
  • name: string
  • created_at: timestamp
• remidnerSchema.jsx
  • title: string
  • description: string
  • due_date: Date, required
  • reminder_date: Date, required