chore: update Prisma and dependencies, add initial migration and tests

- Updated Prisma and @prisma/client to version 6.19.2 in package.json and yarn.lock.
- Added package extensions for @prisma/client in .yarnrc.yml.
- Updated backend README.md for clearer setup instructions.
- Created initial migration for Game, Platform, and related tables in Prisma.
- Added migration lock file for version control.
- Implemented tests for Game model using Vitest, including creation and unique slug constraint checks.

backend/prisma/migrations/migration_lock.toml

@@ -0,0 +1,3 @@
+# Please do not edit this file manually
+# It should be added in your version-control system (e.g., Git)
+provider = "sqlite"