Add Skins as a side game for Bingo Bango Bongo

Players can now opt into a Skins side game when creating a BBB round.
The side game uses the same skins settings (carryover, net/gross, skin
value, double back nine). The scorecard gains a third Skins tab during
play and on the summary; the BBB leaderboard continues to rank by BBB
points.

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

app/database.py

@@ -127,6 +127,7 @@ async def init_db():
             "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",
+            "ALTER TABLE games ADD COLUMN side_skins_enabled INTEGER NOT NULL DEFAULT 0",
         ]:
             try:
                 await db.execute(migration)