fix: restore S3 error stubs, add CI timeouts, gofmt handlers.go, fix comment

- s3.go: restore error returns on prod Upload/Download/Delete stubs
  (silent nil was dangerous — callers would think writes succeeded)
- CI: add -timeout 300s to tests, -timeout 480s to race, GO_TESTING to race env
- handlers.go: gofmt indentation fix for GetCheckoutStatus struct literal
- validators.go: fix comment to include A-F (matched the regex)

.gitea/workflows/ci.yaml

@@ -449,7 +449,7 @@ jobs:
 
       - name: Run tests
         working-directory: backend
-        run: go test -tags "${{ matrix.gotags }}" -count=1 ${{ matrix.verbose }} ${{ matrix.coverflags }} ./...
+        run: go test -tags "${{ matrix.gotags }}" -count=1 -timeout 300s ${{ matrix.verbose }} ${{ matrix.coverflags }} ./...
         env:
           GO_TESTING: "1"
           POSTGRES_HOST: postgres
@@ -531,8 +531,9 @@ jobs:
 
       - name: Run race detector
         working-directory: backend
-        run: go test -tags "${{ matrix.gotags }}" -race -count=1 ./...
+        run: go test -tags "${{ matrix.gotags }}" -race -count=1 -timeout 480s ./...
         env:
+          GO_TESTING: "1"
           POSTGRES_HOST: postgres
           TEST_DB_HOST: postgres
           CGO_ENABLED: "1"