Let anyone with the game URL watch as a read-only spectator

Anonymous users and logged-in non-players can now open /game/<id> and
follow the match: they see the table, the score and each player's
captured cards and scope, but never a hand and never the join code.
Spectator actions are rejected with a forbidden error.

Backend (tavolo-platform / tavolo-scopone):
- /ws/games/{id} accepts anonymous and non-seated connections as
  spectators; only unknown games are refused. The state payload carries
  spectator: true and withholds join_code unless the viewer is seated.
- GET /api/games/{id} is public and returns the same personalized
  (player) or read-only (spectator) view.
- GameEngine.view_for/game_over_view take Optional[str] (None = spectator)
  and are documented to hide privileged information for non-seated
  viewers.

Frontend (Sycamore):
- Parse the spectator flag and render a spectator table: card backs plus
  captured/scope for every seat, no clickable hand, "Spectating" badge.
- A pre-start spectator sees a "game not started yet" waiting room with
  the watch link and no join code.
- "Copy watch link" buttons in the waiting room and the game HUD.
- The hand-end summary carries no acknowledge button for spectators and
  the game-over overlay hides "My matches".

Tests and docs updated.

server/packages/tavolo-scopone/tests/test_plugin.py

@@ -160,6 +160,17 @@ class ViewTest(unittest.TestCase):
         self.assertNotIn("join_code", view)
         self.assertNotIn("game_type", view)
 
+    def test_view_for_spectator_hides_all_hands(self) -> None:
+        engine = ScoponeEngine()
+        session = _started(engine)
+        view = engine.view_for(session, None)
+        for player in view["players"]:
+            self.assertNotIn("hand", player)
+        self.assertNotIn("your_turn", view)
+        self.assertNotIn("legal_moves", view)
+        self.assertIn("table", view)
+        self.assertEqual({"A": 0, "B": 0}, view["scores"])
+
     def test_lobby_view(self) -> None:
         engine = ScoponeEngine()
         session = _session(engine, target_score=16)