fish-cpp: add libgomp1 to runtime image — s2 binary needs OpenMP at runtime

Build succeeded after the libcuda.so symlink fix, but the first
/v1/tts request returned HTTP 500 with:
  s2 binary failed (rc=127): /usr/local/bin/s2: error while loading
  shared libraries: libgomp.so.1: cannot open shared object file

CMake auto-enabled OpenMP during the build (gcc's -fopenmp flag), so
the s2 binary dynamically links libgomp.so.1. The build-stage devel
image had it; the slim cuda:runtime base doesn't ship it by default.
Adding libgomp1 to the runtime image's apt install resolves it.

stacks/fish-cpp/Dockerfile

@@ -50,7 +50,13 @@ ENV DEBIAN_FRONTEND=noninteractive \
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
       python3 python3-pip python3-venv tini \
+      libgomp1 \
     && rm -rf /var/lib/apt/lists/*
+# libgomp1 = GNU OpenMP runtime — required by the s2 binary at runtime.
+# CMake auto-enabled OpenMP at build time (gcc has -fopenmp), so the
+# binary dynamically links libgomp.so.1; the slim cuda:runtime base
+# doesn't include it by default. Without this, s2 fails on first
+# invocation with "error while loading shared libraries: libgomp.so.1".
 
 # Pull the shim deps into an isolated venv so we don't fight system pip.
 RUN python3 -m venv /opt/venv