Fix broken build: pin gradio sdk_version to 5.50.0 for fastrtc compat

Enabling ZeroGPU bumped sdk_version to gradio 6.17.3, which HF injects as a
hard pin into the build. Every fastrtc release requires gradio<6.0, so pip
silently backtracked fastrtc to the 0.0.1 stub (no `Stream`), breaking the
import at runtime. Pin to the latest gradio 5.x (5.50.0) so a current fastrtc
installs, and add a fastrtc>=0.0.20 floor to prevent future backtracking.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -4,7 +4,7 @@ emoji: 📸
 colorFrom: purple
 colorTo: red
 sdk: gradio
-sdk_version: 6.17.3
+sdk_version: 5.50.0
 app_file: app.py
 pinned: false
 license: mit

requirements.txt

@@ -1,4 +1,4 @@
-fastrtc
+fastrtc>=0.0.20
 opencv-python
 twilio
 spaces