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/database.py

@@ -126,6 +126,7 @@ async def init_db():
             "ALTER TABLE magic_link_tokens ADD COLUMN code TEXT",
             "ALTER TABLE games ADD COLUMN skin_value REAL",
             "ALTER TABLE games ADD COLUMN skin_double_back_nine INTEGER NOT NULL DEFAULT 0",
+            "ALTER TABLE games ADD COLUMN skins_net INTEGER NOT NULL DEFAULT 0",
         ]:
             try:
                 await db.execute(migration)