fix: no migration occurs when adding unique true to indexed field (#414)

* feat: alter unique for indexed column

* chore: update docs and change some var names

tests/assets/sqlite_migrate/_tests.py

@@ -18,6 +18,7 @@ async def test_allow_duplicate() -> None:
 async def test_unique_is_true() -> None:
     with pytest.raises(IntegrityError):
         await Foo.create(name="foo")
+        await Foo.create(name="foo")
 
 
 @pytest.mark.asyncio