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/.

buf.gen.yaml

@@ -0,0 +1,11 @@
+version: v2
+plugins:
+  - local: protoc-gen-go
+    out: gen
+    opt: paths=source_relative
+  - local: protoc-gen-go-grpc
+    out: gen
+    opt: paths=source_relative
+  - local: protoc-gen-grpc-gateway
+    out: gen
+    opt: paths=source_relative