Split backend into multiple files

backend/validation.py

@@ -0,0 +1,16 @@
+import re
+
+def is_valid_username(username: str) -> bool:
+    if len(username) < 3 or len(username) > 20:
+        return False
+    if re.search(r'[\s\u180E\u200B-\u200D\u2060\uFEFF]', username):
+        return False
+    return True
+
+
+def is_valid_password(password: str) -> bool:
+    if len(password) < 5 or len(password) > 50:
+        return False
+    if re.search(r'[\s\u180E\u200B-\u200D\u2060\uFEFF]', password):
+        return False
+    return True