Set scopa targets to their traditional defaults

Partnership games go to 21: scopone scientifico (the tournament target)
and scopa a quattro, while two-player scopa keeps its usual 11. The scopa
package resolves the default from the player count and exposes a per
variant options_schema default, so the lobby creation form shows the
right value without frontend changes.

server/tests/test_scopa.py

@@ -99,6 +99,8 @@ class ScopaRoutesTest(unittest.TestCase):
             state = started.json()
             self.assertEqual("playing", state["phase"])
             self.assertEqual(["A", "B", "A", "B"], [p["team"] for p in state["players"]])
+            # Partnership scopa defaults to the traditional 21 points.
+            self.assertEqual(21, state["target_score"])
             for participant in state["players"]:
                 self.assertEqual(3, participant["cards_left"])
             self.assertEqual(4, len(state["table"]))