🎨(backend) simplify boolean comparisons by using opposite operators

Replace inverted boolean comparisons (not ... ==) with direct opposite
operators (!=) to improve code readability and reduce unnecessary
complexity in conditional statements.

src/backend/core/migrations/0014_room_pin_code.py

@@ -17,7 +17,7 @@ def generate_pin_for_rooms(apps, schema_editor):
     def generate_pin_code():
         while True:
             pin_code = str(secrets.randbelow(10 ** length)).zfill(length)
-            if not pin_code in existing_pins:
+            if pin_code not in existing_pins:
                 return pin_code
 
     for room in rooms_without_pin_code: