Improve test coverance and fixes according to tests

app/tests/test_models.py

@@ -0,0 +1,32 @@
+from app.db.models import URL
+
+def test_url_model_defaults(db_session):
+    url = URL(
+        short_code="abc123",
+        original_url="https://example.com",
+    )
+
+    db_session.add(url)
+    db_session.commit()
+    db_session.refresh(url)
+
+    assert url.id is not None
+    assert url.clicks == 0
+    assert url.created_at is not None
+    assert url.last_accessed is None
+
+def test_url_repr(db_session):
+    url = URL(
+        short_code="xyz789",
+        original_url="https://repr-test.com",
+    )
+
+    db_session.add(url)
+    db_session.commit()
+    db_session.refresh(url)
+
+    repr_output = repr(url)
+
+    assert "URL" in repr_output
+    assert "xyz789" in repr_output
+    assert "https://repr-test.com" in repr_output