feat: commit generated Go code, consume via go get instead of submodule

Add go.mod, buf.gen.yaml, and generated gen/kafkamgmt/v1 so consumers
fetch this module directly via go get against the self-hosted Forgejo
host, with no local buf/protoc plugin installation required. Add a
codegen-check CI job to keep gen/ in sync with proto/.

.forgejo/workflows/ci.yaml

@@ -20,3 +20,26 @@ jobs:
       - name: buf breaking (against main)
         run: buf breaking --against '.git#branch=main'
         if: github.ref != 'refs/heads/main'
+
+  codegen-check:
+    runs-on: docker
+    container:
+      image: golang:1.25
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install buf and protoc plugins
+        run: |
+          go install github.com/bufbuild/buf/cmd/buf@latest
+          go install google.golang.org/protobuf/cmd/protoc-gen-go@latest
+          go install google.golang.org/grpc/cmd/protoc-gen-go-grpc@latest
+          go install github.com/grpc-ecosystem/grpc-gateway/v2/protoc-gen-grpc-gateway@latest
+
+      - name: Regenerate and diff
+        run: |
+          export PATH="$PATH:$(go env GOPATH)/bin"
+          buf generate
+          git diff --exit-code -- gen || (echo "::error::gen/ is out of date — run buf generate and commit the result" && exit 1)
+
+      - name: go build
+        run: go build ./...