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/packages/tavolo-scopone/README.md

@@ -19,7 +19,8 @@ implementation.
 - `plugin.py` — `ScoponeEngine(GameEngine)`: the platform-facing adapter.
   It translates create/join/websocket actions/deadlines into rules-engine
   calls and back, validates the `target_score`/`napola`/`asso_pigliatutto`
-  creation options, and extracts the `MatchResult` (teams, winner,
+  creation options (the match target defaults to the traditional 21
+  points), and extracts the `MatchResult` (teams, winner,
   per-player scores and the match summary persisted as the match's JSON
   `result`).