Add skin value and back-nine doubling to skins games

Players can now set a monetary value per skin when starting a skins game.
For 18-hole games, an option to double the value on holes 10–18 is available.
The value is stored at game creation and displayed throughout the live game
and summary (leaderboard, scorecard, and totals).

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

app/database.py

@@ -124,6 +124,8 @@ async def init_db():
             "ALTER TABLE game_players ADD COLUMN playing_handicap INTEGER",
             "ALTER TABLE game_players ADD COLUMN strokes_allocation TEXT",
             "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",
         ]:
             try:
                 await db.execute(migration)