Revert to gradio 5.9.1 (4.x broken with newer jinja2/huggingface_hub)

Fix: gradio 4.44 + pin huggingface_hub==0.23.0 to avoid HfFolder error

Fix: remove gradio from requirements (SDK manages it via sdk_version)

Fix: use gradio 5.9.1 (HfFolder removed in newer huggingface_hub)

Fix: downgrade sdk_version to 4.44.0, python to 3.11 to unblock Space build

Add prompt injection detector (protectai/deberta-v3-base-prompt-injection-v2)