fix: correct VideoPrism import (videoprism.models, not videoprism); add flax dep

videoprism/__init__.py is empty — API lives in videoprism.models.
Fix: from videoprism import models as vp (not import videoprism as vp).
Also add flax to managed venv packages (required by videoprism Flax model).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nodes/feature_extractor.py

@@ -18,7 +18,7 @@ _EXTRACT_PACKAGES = [
     # TF 2.15 only supports Python <=3.11; use >=2.16 for Python 3.12+
     "tensorflow-cpu>=2.16.0",
     # jax[cuda13] includes jaxlib; pip-managed CUDA libs (no local toolkit needed)
-    "jax[cuda13]",
+    "jax[cuda13]", "flax",
     "transformers", "decord", "einops", "numpy", "mediapy",
     "git+https://github.com/google-deepmind/videoprism.git",
 ]