feat: add VideoPrism temporal video search

[tulayha]

Summary

• Add VideoPrism as a first-class temporal action-search capability shared by CLI, HTTP, MCP, and local worker paths.
• Pin and prepare only the PyTorch VideoPrism checkpoint, expose temporal clip controls and search in Streamlit, and include the capability in desktop/runtime dependency metadata.
• Flatten VideoPrism's pooled video and text outputs before Chroma storage and retrieval.
• Existing indexes remain compatible, but media must be re-indexed with VideoPrism selected to use temporal search. The prepared model adds approximately 0.93 GiB to the local model cache.

Validation

• uv run --no-sync pytest -q tests/test_videoprism.py — 4 focused capability tests passed.
• uv run --no-sync pytest -q tests/test_frontend.py tests/test_frontend_app.py — 32 frontend tests passed.
• uv run --no-sync ruff check src/vidxp/capabilities/videoprism tests/test_videoprism.py — capability lint passed.
• Manual local boundary: prepared the pinned VideoPrism model, launched uv run --no-sync vidxp ui, and produced an active VideoPrism snapshot through Streamlit.