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-scopa/README.md

@@ -23,9 +23,11 @@ The optional *asso pigliatutto* creation option turns either variant into
   (four players in fixed partnerships): the platform-facing adapters.
   They translate create/join/websocket actions/deadlines into
   rules-engine calls and back, validate the
-  `target_score`/`asso_pigliatutto` creation options, and extract the
-  `MatchResult` (teams, winner, per-player scores and the match summary
-  persisted as the match's JSON `result`).
+  `target_score`/`asso_pigliatutto` creation options (the match target
+  defaults to the traditional 11 points for two-player scopa and 21 for
+  scopa a quattro), and extract the `MatchResult` (teams, winner,
+  per-player scores and the match summary persisted as the match's JSON
+  `result`).
 
 Timeouts are constructor arguments (`turn_timeout_seconds`,
 `hand_ack_timeout_seconds`), wired from the environment by the