Add Course model validations and missing fields

[deepsonK]

File: backend/models/Course.js\n\nIssue: Missing validations, unique constraints, and room field\n\nDifficulty: Medium\n\nWhat to add:\n1. Add unique: true to courseCode\n2. Add room/location field\n3. Add capacity validation\n4. Make professor required\n5. Add course number validation