Add FFmpeg setup step to Gitea workflow for enhanced media processing

- Included a new step in the test-pr.yaml workflow to set up FFmpeg, improving the project's media handling capabilities.
- This addition complements the existing build steps for Go and frontend assets, ensuring a more comprehensive build environment.

.gitea/workflows/test-pr.yaml

@@ -10,6 +10,7 @@ jobs:
       - uses: actions/setup-go@main
         with:
           go-version-file: 'go.mod'
+      - uses: FedericoCarboni/setup-ffmpeg@v3
       - run: go mod tidy
       - run: cd web && npm install && npm run build
       - run: go build ./...