Use upload-artifact@v3 — v4 not supported by Gitea Actions

upload-artifact@v4 relies on the new @actions/artifact 2.x protocol and
fails on Gitea with GHESNotSupportedError. v3 uses the older artifact
API that Gitea implements.

.gitea/workflows/build-apk.yml

@@ -75,13 +75,15 @@ jobs:
         run: flutter build apk --release
 
       - name: Upload debug APK
-        uses: actions/upload-artifact@v4
+        # Gitea Actions supports only the v3 artifact protocol;
+        # upload-artifact@v4 fails with GHESNotSupportedError.
+        uses: actions/upload-artifact@v3
         with:
           name: onbudget-debug-apk
           path: build/app/outputs/flutter-apk/app-debug.apk
 
       - name: Upload release APK
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v3
         with:
           name: onbudget-release-apk
           path: build/app/outputs/flutter-apk/app-release.apk