fix(qwen-tts): install flash-attn on CUDA 13

Add the missing cublas13 post-install requirements hook so the CUDA 13 backend uses FlashAttention instead of falling back to SDPA. Keep CUDA packaging parity covered by the backend test script.

Assisted-by: Codex:gpt-5

Author: localai-org-maint-bot

backend/python/qwen-tts/requirements-cublas13-after.txt

@@ -0,0 +1 @@
+flash-attn

backend/python/qwen-tts/test.sh

@@ -2,6 +2,11 @@
 set -e
 
 backend_dir=$(dirname $0)
+
+for cuda_version in 12 13; do
+    grep -qx "flash-attn" "$backend_dir/requirements-cublas${cuda_version}-after.txt"
+done
+
 if [ -d $backend_dir/common ]; then
     source $backend_dir/common/libbackend.sh
 else