fix(backend): add missing matplotlib dependency for podcast composer

The podcast B-roll composer imports matplotlib for chart rendering, so adding it to backend requirements prevents import failures in fresh setups.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/requirements.txt

@@ -45,6 +45,7 @@ numpy>=1.24.0
 
 # Image/media for podcast
 Pillow>=10.0.0
+matplotlib>=3.8.0
 huggingface_hub>=1.1.4
 
 # TTS for podcast