Add straight up / net handicap option for skins games

When creating a skins game, players can now choose between straight up
(gross scores) or net scoring (strokes subtracted per hole based on
playing handicap and stroke index). Net mode is stored on the game and
applied in both the live scorecard and the summary.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/templates/games/summary.html

@@ -7,7 +7,7 @@
     <a href="javascript:history.back()" style="color:var(--text-muted); font-size:0.9rem;">← Back</a>
     <div style="text-align:center;">
       <div style="font-weight:600;">{{ game.course_name or 'Bingo Bango Bongo' }}</div>
-      <div style="font-size:0.8rem; color:var(--text-muted);">{{ game.holes_count }} holes · {{ game.created_at[:10] }}</div>
+      <div style="font-size:0.8rem; color:var(--text-muted);">{{ game.holes_count }} holes · {{ game.created_at[:10] }}{% if game.format == 'skins' and skins_net %} · net{% endif %}</div>
     </div>
     <div style="width:3rem;"></div>
   </div>