fix(prisma): track migration SQL files in git

migration.sql was gitignored, so CI builds a Docker image from a
fresh clone with empty migration directories. prisma migrate deploy
then has nothing to apply, leaving the database schema-less on first
launch (P2021: table does not exist).

prisma/migrations/20260729184000_add_activity_location/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "ActivityOption" ADD COLUMN "location" TEXT;