Improve test coverance and fixes according to tests

app/tests/test_utils.py

@@ -0,0 +1,19 @@
+import string
+from app.utils.short_code import generate_short_code
+
+def test_generate_short_code_length():
+    code = generate_short_code(8)
+    assert len(code) == 8
+
+def test_generate_short_code_is_alphanumeric():
+    code = generate_short_code(12)
+    allowed = string.ascii_letters + string.digits
+
+    for char in code:
+        assert char in allowed
+
+def test_generate_short_code_randomness():
+    code1 = generate_short_code()
+    code2 = generate_short_code()
+
+    assert code1 != code2  # extremely unlikely to fail