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.

.yarnrc.yml

@@ -1 +1,6 @@
 yarnPath: .yarn/releases/yarn-4.12.0.cjs
+
+packageExtensions:
+  "@prisma/client@*":
+    dependencies:
+      ".prisma": "*"