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.

.gitea/workflows/ci.yaml

@@ -38,7 +38,7 @@ jobs:
         run: yarn workspace quasar-backend run lint
 
       - name: Run backend unit tests
-        run: yarn workspace quasar-backend run test
+        run: yarn workspace quasar-backend run test:ci
 
   e2e:
     runs-on: ubuntu-latest